11a (clue "see rook in a tree from seat") - B-r(ook)-eech
17a Alphabetical - as the letters in "chimp"
20a Homonyms - same sound, different meaning
23a No chance (cards, game of Monopoly)
12d Endeavoured - 'E(nglish)' + 'n' (knight, chess) + 'devoured' (outside) 'a(ssizes)' (front of)