Thanks to Raich for this thematic puzzle. Definitions are underlined in the clues.
The crossword marks the 80th anniversary today of the signing of Munich Agreement, with the solutions to 1 across, 11 across, 15 across, 19 across, 24 across, and 29 across.
Across
1 Uproar! Met foe, ice broken – so 11 said (5,3,3,4)
PEACE FOR OUR TIME : Anagram of(… broken) UPROAR! MET FOE, ICE.
Defn: So declared Neville Chamberlain;answer to 11 across on 30 September 1938, less than a year before war broke out.
9 I work for Independent, say, in Shanghai? (7)
IMPRESS : [ I’M PRESS](I am;I work for the press, of which The Independent newspaper is an example;say).
Defn: …;to force;press;impress someone to join a ship’s crew, an activity that used be conducted by press gangs (not gangs of reporters, though).
10 Large place is useful at first, area for redevelopment (7)
EURASIA : Anagram of(… for redevelopment) [IS + 1st letter of(… at first) “useful” + AREA].
11 Hall, city home for leader (11)
CHAMBERLAIN : CHAMBER(a hall;a large room used for formal or public events) + LA(abbrev. for the US city of Los Angeles) + IN(at home).
Defn: …;former Prime Minister of the UK.
13 Cunning, finding time to supplant king in vessel (3)
ART : “t”(abbrev. for “time”) replacing(to supplant) “k”(abbrev. for “king” in chess notation) in “ark”(a vessel, like the one built by the biblical Noah).
Defn: …, as a noun.
14 Sir’s date worked out badly? (8)
DISASTER : Anagram of(… worked out) SIR’S DATE.
Defn: A situation (eg. that knight’s date) that worked out badly;resulted in a disaster.
15 Leader‘s girl’s accommodating Italian learner (6)
HITLER : HER(the feminine possessive pronoun;girl’s) containing(accommodating) [IT(abbrev. for “Italian”) + L(letter displayed by a learner driver) ].
Defn: …;Führer of Nazi Germany.
19 Champ embracing one in conference centre (6)
MUNICH : MUNCH(to champ;to chew noisily) containing(in) I(Roman numeral for “one”).
Answer: The venue, since 1963, of the Munich Security Conference, an annual conference on international security. It was also the venue in 1938 of the negotiations (less of a conference) amongst Germany, France, the UK, and Italy that resulted in the Munich Agreement.
21 Spinning stroke from bounder, cute (8)
UNDERCUT : Hidden in(from) “bounder, cute“.
Defn: A strike on a ball with a downward glancing blow to give it a backspin.
23 Part of match fixed by agreement (3)
SET : Double defn: 1st: …, a tennis match, that is; and 2nd: …;arranged in advance after an agreement is reached.
24 Suitable to include vegetables provided by English chaps – 11’s policy (11)
APPEASEMENT : APT(suitable;fitting) containing(to include) [ PEAS(vegetables) plus(provided by) E(abbrev. for “English”) + MEN(chaps) ].
26 A mother encounters worker, unyielding (7)
ADAMANT : A + DAM(the mother of an animal, usually a domestic mammal) + ANT(a social insect, of which one caste are the workers).
27 Joke after warning needed for cut of beef (7)
FORERIB : RIB(to joke;to tease) placed after(after) FORE!(called out as a warning to people in the path of a golf ball).
29 Uphill feel found unfortunately – fate of 1 Across (11,4)
UNFULFILLED HOPE : Anagram of(… unfortunately) UPHILL FEEL FOUND.
Defn: What the solution to 1 across turned out to be;what its fate was.
Down
1 Area noble rules, proper, accepting respectable party I overlooked (9)
PRINCEDOM : PRIM(proper;very formal and correct) containing(accepting) “nice”(respectable;refined in language, manners, etc, as in “nice people don’t do that!”) + “do”(a party;a social function) minus(… overlooked) “I“.
2 A very quiet listener, silent initially, comes into view (7)
APPEARS : A + PP(abbrev. for “pianissimo”, the musical direction to play very softly;quietly) + EAR(what one listens with;a listen-er) + 1st letter of(… initially) “silent“.
3 Not entirely impartial girl (3)
EVE : “even”(impartial, as in “to give even treatment to both sides”) minus its last letter(Not entirely…).
4 One watching outside broadcast with restaurant worker (8)
OBSERVER : OB(abbrev. for “outside broadcast”, a radio or television programme that is broadcast from a venue other than the radio or television studio) plus(with) SERVER(a restaurant worker who serves food to the diners).
5 Last of home gatherings (5)
OMEGA : Hidden in(of) “home gatherings“.
Defn: … of any series;the ending.
6 Referring to game at Twickenham, nation’s latest, that’s shown again (5)
RERUN : RE(with referenceto;referring to) RU(abbrev. for “rugby”, the game that Twickenham Stadium, London was built for) + last letter of(…’s latest) “nation“.
7 Put into office wearing seconds? Hard to believe! (7)
INSTALL : IN(wearing;clothed in…) S(abbrev. for “seconds” in time notation) + TALL(hard to believe, such as “a tall tale”).
8 Precise court overturning dismissal at first (5)
EXACT : CT(abbrev. for “court”) placed below(… at first, in a down clue) reversal of(overturning, in a down clue) AXE(dismissal;cancellation, as in “half the department is facing the axe”).
12 Simple article is written up – set in ancient times (5)
BASIC : [ A(an article in grammar) + reversal of(… written up, in a down clue) IS ] contained in(set in) BC(abbrev. for “Before Christ”, or “before the birth of Christ”, indicating ancient times).
16 French department‘s unusual soiree, nothing missed out (5)
ISERE : Anagram of(unusual) “soiree” minus(… missed out) “o”(letter signifying 0;nothing).
Defn: An administrative region;department in eastern France.
17 After climbing a hill, board can be brought around (9)
ROTATABLE : Reversal of(After climbing, in a down clue) [A + TOR(a hill)] + TABLE(board;food provided in a household).
18 Forbidden, a French novel ultimately unpleasant (8)
UNLAWFUL : UN(French for “a”) + last letter of(… ultimately) “novel” + AWFUL(unpleasant;bad).
20 Tart brought up tho crushed inside? Indeed! (3,4)
NOT HALF! : Reversal of(… brought up, in a down clue) FLAN(a tart;an open-topped pastry case with a savoury or sweet filling) containing(… inside) anagram of(… crushed) THO.
Defn: …;undoubtedly!
22 He meets rising anger in firm for so long (7)
CHEERIO : [HE plus(meets) reversal of(rising) IRE(anger) ] contained in(in) CO(abbrev. for “company”;a commercial firm).
Defn: …;farewell!
23 Chaos upset United supporters (5)
SNAFU : Reversal of(upset, in a down clue) [ U(abbrev. for “united”) + FANS(supporters, possibly of football teams named “such and such United”).
Defn: From the acronym for “Situation Normal, All F***ed Up”, or “… Fouled Up”, if you prefer).
24 A part of nun’s clothing, we hear, to be of use (5)
AVAIL : A + homophone of(…, we hear) “veil”(part of a nun’s clothing).
Defn: To make use of;to take advantage of an opportunity or available resource.
25 Boys painted place, it’s looking up (5)
PUTTI : PUT(to place in a particular position) + reversal of(…’s looking up, in a down clue) IT.
Defn: … in Renaissance art.

28 Radical friend regularly visited (3)
RED : 2nd, 4th and 6th letters of(… regularly visited) “friend“.
Thanks scchua and to Raich for a timely and thought provoking puzzle.
I think 14 probably qualifies as thematic.
After ‘Hitler’ and ‘Appeasement’ the light dawned. A nice anagram at 1ac, spent a while looking for the N as I remembered it as Peace IN our time. Never happy with the 2 Ls in Install, but as an ex-IT bloke I saw it often enough to have to get over it. Thanks to Raich and Scchua.
Starting on the acrosses the only ones we got on our first pass were HITLER and ADAMANT, and we wondered what the former was doing there. Then we started on the downs and the anagram in 1ac shook itself out and we were well away, realising that it must be the anniversary of that infamous event.
A sobering theme, but plenty of non-thematic clues to appreciate. We liked FORERIB, INSTALL, UNLAWFUL and NOT HALF.
Thanks, Raich and scchua
Many thanks scchua for the excellent blog and thanks to all who commented. In 19, was not thinking beyond the 1938 meeting: 14 not intended as thematic but when it got into the grid thought it might be seen as such. Thanks to eimi for publishing it 80 years to the day since Chamberlain waved the piece of paper to ecstatic crowds in London on his return there.