Thank you to Mudd. Definitions are underlined in the clues.
Across
1. Poultry farm’s hierarchy? (7,5)
PECKING ORDER : Cryptic defn: … of members in a group according to status or power, so-called from the way such order is maintained by domestic fowl, such as in a poultry farm – by a higher bird pecking others lower down.
10. Hole, if between gold and diamonds (7)
ORIFICE : IF contained in(between) [ OR(the colour gold/yellow in heraldry) plus(and) ICE(slang for the precious stones, diamonds) ].
11. Rubbish to empty out tomorrow before decay (7)
TWADDLE : Inner letters deleted from(empty out) “tomorrow” plus(before) ADDLE(to decay/to rot).
12. Criticise achievement of batter (5)
KNOCK : Double defn: 2nd: An innings or spell at batting achieved by a batsman/batter? in cricket, or, simply, a blow accompanied by a loud sound made by anyone with a bat.
13. Moderate break before fall (8)
RESTRAIN : REST(a break/a pause during proceedings) placed before(before) RAIN(to fall/precipitate in large quantities).
Defn: …/make less extreme.
15. Departure from the norm a bit rare, on changing (10)
ABERRATION : Anagram of(… changing) A BIT RARE, ON.
16. Page — one has a margin (4)
LEAF : Double defn: 1st: A … in, say, a book; and 2nd: Part of a plant or tree that is flat and usually green, and whose edge is called a margin.
Ant of course, a page with printed text also has a margin or margins.
18. Grey matter — might one be white or brown? (4)
LOAF : Double defn: 1st: Slang for one’s brain/grey matter/intelligence; and 2nd: A shaped lump of white or brown bread.
20. Person taking photos of a communist swimmer (3,7)
RED SNAPPER : Possible description of a SNAPPER/person taking photos/snaps of a RED/a communist.
22. Go towards the cellar perhaps for refuse (4,4)
TURN DOWN : TURN(go/the opportunity for one to act, as in “it’s your go after the dealer has dealt all the cards”) + DOWN(towards the cellar/direction to go the room below ground level)
24. PM fell, say (5)
HEATH : Double defn: 1st: Former UK Prime Minster; and 2nd: …/stretch of moorland. (Not sure how “say” fits in).
26. Chopper heading for Cardiff, one departing (7)
CLEAVER : 1st letter of(heading for) “Cardiff” + LEAVER(one departing/leaving).
27. Setter for example reflected on classical language (7)
GELATIN : Reversal of(… reflected) EG(abbrev. for “exempli gratia”/for example) plus(on) LATIN(classical language).
Defn: …/a compound that thickens liquids, used in eg. food.
28. OK laid across ground, very secure (5,2,1,4)
SOLID AS A ROCK : Anagram of(… ground) OK LAID ACROSS.
Down
2. Chapter torn from very long book, perfect example (7)
EPITOME : “epic tome”(a very long book) minus(… torn from) “c”(abbrev. for “chapter”).
3. Oh no underwear! (8)
KNICKERS : Double defn: 1st: …!/an expression of annoyance.
4. Requirement in fine editorial (4)
NEED : Hidden in(in) “fine editorial“.
5. Where chimney pot may be found for nothing (2,3,5)
ON THE HOUSE : Literally, where a chimney pot may be found in one’s home.
6. Crackers filled with last of cheddar, rough (5)
DRAFT : DAFT(crackers/insane) containing(filled with) last letter of(last of) “cheddar“.
Defn: … as a noun/a preliminary sketch.
7. Admin regulations in dept are ludicrous (3,4)
RED TAPE : Anagram of(… ludicrous) DEPT ARE. And they are.
8. Mark found under bird tracks — that’s a fine thing! (8,5)
COCKTAIL STICK : TICK(a mark/a check mark) placed below(found under, in a down clue) [ COCK(a male bird) + TAILS(tracks/follows) ].
In cocktails:
… and in cocktail sausages: 
9. Ordinary old coins (5-8)
PENNY-FARTHING : PENNY,FARTHING(two old coins of British currency).
Defn: English name for the bicycle with one big and one small wheel, called an ordinary in America.
named after these: 
14. Troubling row with the Lord in the afterlife (10)
OTHERWORLD : Anagram of(Troubling) [ROW plus(with) THE LORD ].
17. Composer with part written up for graduate (8)
BACHELOR : BACH(Johann Sebastian, German composer) plus(with) reversal of(… written up, in a down clue) ROLE(a part played in a play or film).
Defn: Short for a fist degree holder/a graduate.
19. Stuffy serials broadcast (7)
AIRLESS : Anagram of(… broadcast) SERIALS.
21. Image picking up previous credit (7)
PLASTIC : PIC(short for “picture”/an image) containing(picking up) LAST(previous/before the current one).
Defn: Short for “plastic money”/credit cards used to purchase items on credit instead of with cash.
23. Satan existed, looking from the abyss? (5)
DEVIL : Reversal of(…, looking from the abyss ie. looking up from down below, in a down clue) LIVED(existed).
25. Long time in embrace of scallywag especially (4)
AGES : Hidden in(in embrace of) “scallywag especially“.
Well, I never knew that a penny farthing is also an ordinary. And not just in the US, according to Wikipedia.
And thanks scchua for explaining LEAF, the second meaning for which I was unaware.
Apart from one or two sticky clues at the end, I found this very easy, but enjoyable nonetheless. Thanks Mudd.
What a good puzzle with a mixture of some remarkably easy as well as more tricky clues with a fun blog. Like CDU I did not know the bicycle meaning of ordinary.
Thanks to Mudd and Scchua
Thanks both.
3d KNICKERS was the answer to the first clue in Paul’s first crossword in the Guardian.
A pretty quick solve with some very easy clues and then a few tricky ones.
I don’t like clues which require obscure general knowledge, like knowing that an ordinary is a Penny Farthing in the USA. Simply impossible to solve from the definition bit for most of us.
I felt the definition of cocktail stick was also weak.
Loi was Loaf
Excellent blog – thank you.
And thank you Mudd.
12A: as usual I had no idea of the cricket reference, so solved this only by the first definition and the cross letters.
3D: again solved only by the cross letters and the reference to “underwear”. I have never heard this word as a meaning of “Oh no”.
9D: I guessed the two coins but didn’t know the US version of the old bike.
I’m with Moly re 8D. Is a “fine thing” really a suitable definition of a “cocktail stick”? That definition could also include a parking ticket or a piece of thread.
Same as Moly, SM and GDU
Additionally, my start to crosswords was via the FT weekend when there was a Mudd every fortnight, so I must confess a certain fondness for Mudd and am always happy to see his name. Favourites were GELATIN for its surface, SOLID AS A ROCK for the anagram, and the cheeky OTHERWORLD
Thanks Mudd and scchua
Great pics in blog scchua. This whizzed along although I did find myself staring at P-A-T-C for ages trying to make it be something like INSTAMATIC. PLASTIC forced itself into my head so eventually I put it in and was surprised to find that it was right. Liked BACHELOR for the musical aspect.
Thanks for the blog, very enjoyable puzzle. Bracoman@3 I remember doing that puzzle on the day and thinking it was a bit different, Paul used to be far more smutty for the Guardian.
Peter @5 , saying knickers is a bit like drat, a mild oath when something goes wrong such as knocking a cup of tea over.