Across
1 POSTGRAD G(ood) substituted for the ‘e’ in Poster, being one of the two advertisements, the other being ‘ad.
5 Not being interested in football I thought long and hard about Ferdinand the Bull, until I saw RIO reversed into SEE for SOIREE
9, 23 COCKTAIL PARTY being an AI LP (excellent record) inside a bird, COCK, and provocative – TARTY
10 THRASH - a party and to beat.
12 LEAVE using ‘a’ for one inside a curtailed levee.
13 HANSEATIC I knew about the Hanseatic League from a brief spell working in Hamburg. They were a monopolistic union of traders around the Baltic between the 13th and 17th centuries.
14 P ITCHED ROOFS
18 SWAG GERS TICK gers is a French region, and tick is credit.
21 A CHIL LEAN ‘chil’ is the reduced cold, lean is spare, and Yarrow is Achillea with ‘N’ standing for name. A complicated set of hints and satisfying to solve.
24 REMOVE I spent a long time thinking it would end in T(ime) before spotting remove buried in ‘haREM OVEr time.
25 C LOIS T ER
26 I googled The Warden, not having read it, before spotting the anagram of warden – ANDREW Saint Andrew is well known, and Merry Andrew was a buffoon, or clown.
27 HYP NO SIS A joiner is a hyphen, take off the hen, and no sis(ter).

Down
1 PICKLE
2, 3 Unfortunately I got 15 late in the crossword, so that was no help. Then I saw the anagram for cosi, gala, the ring (Wagner’s work) and it all fell into place, and my solving speeded up.
4 A TIGHT CORNER is a predicament and I was in one until stopped worrying about the board game and realised a corner in a market is a monopoly.
6 OCH RE
8 ETHICISM – anagram it chimes
11 IN TEST IN ALLY ‘in’ here means batting, in cricket.
15 I realised it was RECEPTION long before I made the connection with the first class at school, which is now generally known as reception.
16 A SPAT RIA – a place in Cumbria (and yes, it is, I’ve checked.)
17 F AT HOME D The man swinging the lead is checking the depth of water under a boat in fathoms, hence fathomed meaning to work out FD stands for Fidei Defensor (Defender of the Faith), and is around an at home – a social gathering.
19 G RAT IS, GIs are American soldiers, and a rat is a deserter.
22 LEVEE An embankment against flooding, and also a reception for someone important.