Thanks to Peter for an enjoyable puzzle. Definitions are underlined in the clues.
Across
1 Matches at Chelsea entertaining one carrying books? (7)
SATCHEL : Hidden in(… entertaining) “Matches at Chelsea“.
5 Symbol in text is ring over loop (7)
DINGBAT : DING(to make a ringing sound like a bell) + reversal of(over) TAB(a loop;a small flap of material attached to something to hold or fasten the latter). D0 … other than letters or numerals, indicating divisions or replacing letters.

9 Dummy swapping diamonds for time in temporary accommodation (5)
MOTEL : “model”(a dummy, in a shop window modelling clothing, say) with “d”(abbrev. for the suit of diamonds in card games) replaced by(swapping … for …) “t”(abbrev. for “time”).
10 Medical examination for relative with second pulse (9)
MAMMOGRAM : MAM(informal term for one’s mother, a relative) plus(with) MO(short for “moment”, such as a second, as in “wait a moment”) + GRAM(chickpeas or other pulses used as food).
11 Second attempt to get hydrogen and nitrogen into surprisingly nice poison (10)
STRYCHNINE : S(abbrev. for “second” in time notation) + TRY(an attempt) plus(to get) [ H(symbol for the element hydrogen) plus(and) N(symbol for the element nitrogen) ] contained in(into) anagram of(surprisingly) NICE.
12 Dog in river leaving scent (4)
TAIL : “trail”(a scent used in following someone or hunting an animal) minus(… leaving) “r”(abbrev. for “river”).
Defn: To follow after someone closely and persistently.
14 Potter’s electronic tool? (5,7)
SPELL CHECKER : Cryptic defn: Reference to Harry Potter, the fictional character who casts spells, and presumably would use some sort of tool to check them out.
Defn: A computer programme that checks the spelling of text inputs.
18 Urban set near newly built underground (12)
SUBTERRANEAN : Anagram of(… newly) URBAN SET NEAR.
21 Whirlwind decapitated cuddly toy (4)
EDDY : “teddy”(short for a teddy bear, a cuddly stuffed toy) minus its 1st letter(decapitated …).
22 Spring cured so-called rich (4-6)
WELL-HEELED : WELL(a spring;a source of water, say, from underground) + homophone of(… so-called) “healed”(cured of a disease, say).
25 Flavouring made from small fruit in perfect condition (9)
SPEARMINT : S(abbrev. for “small”) + PEAR(a fruit) + MINT(in perfect condition;unused).
Defn: The common garden mint from which is derived the flavouring of the same name.
26 Force independent politician to meet the Spanish (5)
IMPEL : I(abbrev. for “Independent”, describing a candidate who belongs to no particular political party) + MP(abbrev. for a Member of Parliament, an elected politician) plus(to meet) EL(Spanish for “the”).
27 Queen stopping King Edward, say, being sent back to refuge (7)
RETREAT : Reversal of(…, being sent back) [ ER(abbrev. for Elizabeth Regina, the Queen) contained in(stopping) TATER(informal term for “potato”, a variety of which;say, is the King Edward) ].
28 Priest in newly sealed container slipped away (7)
ELAPSED : P(abbrev. for “priest”) contained in(in … container) anagram of(newly) SEALED.
Down
1 Prophet tucked into South American snack (6)
SAMOSA : AMOS(a Minor Prophet) contained in(tucked into) SA(abbrev. for “South American”).

2 Head of troubled university going to sort out staff (6)
TUTORS : 1st letter of(Head of) “troubled” + U(abbrev. for “University”) plus(going to) anagram of(… out) SORT.
Defn: … at a learning institution, a university with problems, say. Nice surface.
3 Hitler and Luftwaffe finally prepared to keep officer in aircraft (10)
HELICOPTER : Anagram of(… prepared) [HITLER plus(and) last letter of(… finally) “Luftwaffe“] containing(to keep) COP(informal term for a police officer).
4 Colour of worthless item (5)
LEMON : Double defn: 2nd: …, such as a new car that gives numerous problems.
5 West Indian academic taking car to California (9)
DOMINICAN : DON(an academic at a university) containing(taking) [ MINI(the iconic British car, first produced in the 60s) plus(to) CA(abbrev. for the US state of California) ].
Defn: … from the Dominican Republic.
6 Beginner returning favour … (4)
NOOB : Reversal of(returning) BOON(a favour or request asked of someone).
Defn: One who is inexperienced in a particular activity, especially in computing or Internet usage.
One of those clues that requires a checking letter to confirm which end of the clue is the definition.
7 … Supports housing a minister in military accommodation (8)
BARRACKS : BACKS(supports) containing(housing) [ A + RR(abbrev. for “Right Reverend”, a title for certain religious figures, or ministers) ].
8 Note on mobile model about start of regeneration for tardis user (4,4)
TIME LORD : TI(the seventh note of a major scale in music) placed above(on, in a down clue) anagram of(mobile) MODEL containing(about) 1st letter of(start of) “regeneration“.
Defn: What Dr. Who – the TV series character with his time machine, the TARDIS – was. It’s now a she-lord and her time machine:

13 Sage mixes a cocktail for second Sunday in Lent (10)
SEXAGESIMA : Anagram of(… cocktail) SAGE MIXES A.
Defn: …, the Christian religious observance period.
15 Liberal listened to criminal and thief (9)
LARCENIST : L(abbrev. for “Liberal”, one belonging to the Liberal Party) + homophone of(listened to) “arsonist”(a criminal who deliberately sets fire to property).
16 Fools mostly stood up for judge (8)
ASSESSOR : ASSES(fools;human forms of those reputedly dim animals) + reversal of(… up, in a down clue) “rose”(stood up from a lying, sitting or kneeling position) minus its last letter(mostly …).
17 Good decoration on stamp books (8)
OBEDIENT : OBE(abbrev. for “Officer of the Most Excellent Order of the British Empire”, an honour or decoration given to individuals by the Queen) placed above(on, in a down clue) DIE(a stamp;a device for moulding metal into a specific shape) + NT(abbrev. for the New Testament, the collection of books in the Bible).
Defn: …, in terms of following orders.
19 Central ingredient in bread featuring in food waste lists (6)
SLOPES : Middle letter of(Central ingredient in) “bread” contained in(featuring in) SLOPS(food waste).
Defn: …;leans to one side.
20 Confused clergyman taken in by dodgy deal (6)
ADDLED : DD(abbrev. for “Doctor of Divinity”, a degree conferred on outstanding religious scholars, whom, I presume, most clergymen are) contained in(taken in by) anagram of(dodgy) DEAL.
23 Furious athlete losing three extremities in revolving machine (5)
LATHE : Anagram of(Furious) [ “athlete” minus 1st and last letters of(losing … extremities) “three” ].
24 Poacher nearly catching bird (4)
ERNE : Hidden in(… catching) “Poacher nearly“.
Found this harder than her usual offerings but all the more enjoyable for that.
Didn’t know SEXAGESIMA, but, fortunately, this was my first guess for the anagram once I had all the crossers. SLOPES was my LOI.
Wasn’t sure about “stood” to mean “rose” in 16d. “Stood up” works but then “up” is doing double duty, something I don’t like. Thinking about it as I type, “He stood to attention” seems to work.
In 10a, I didn’t know that meaning for “gram”. Looking it up, I see it can also mean a village in India.
Thanks to Peter and scchua.
Enjoyable fare for a sunny Sunday morning.
I came across GRAM as a pulse only last week when I was trying to find out what SEV was ( it’s made from chickpea flour paste and was delicious). Very pleased with myself for remembering SEXAGESIMA even if I needed help in spelling.
Thanks to setter and blogger.
We found this enjoyable too. We had no problem with ‘stood’ for ‘rose’; an audience or a congregation may be asked to stand or the entrance of a dignitary – whereupon they will rise (from their seats). But we thought 9ac was ambiguous and could have given ‘model’ or ‘motel’, though of course 2dn settled the issue.
No real CoD but we liked SPELL CHECKER, HELICOPTER and LARCENIST.
Thanks, Peter and scchua.
Having got stuck on this week’s Azed, I thought I’d put it to one side and try this. Went in surprisingly quickly with little in the way of problems. 13dn took me back to having diaries that told me a Sunday was Sexagesima (the week after Septuagesima, I see).