two words(with only 1 common letter) in the clue need to be switched...
20a Poet's left uncut verse's conclusion following moderation (4) HOL?
HOLE? .....defn?
The letter from the switched words is i
''Poet's [uncut][left] verse's conclusion following moderation'' (the letter is 't') -
Poet's uncut' is the def for 'hole' (archaic spelling = 'whole' for Spenser). L+ 'vers(E') 'following' HO = 'moderation'.