Sees boozers gathered round the piano (5)
I believe the answer is:
spots
'sees' is the definition.
(I've seen this before)
'boozers gathered round the piano' is the wordplay.
'boozers' becomes 'sots' (I've seen this before).
'gathered round' is an insertion indicator.
'the piano' becomes 'p'.
'sots' going around 'p' is 'SPOTS'.
(Other definitions for spots that I've seen before include "Espies the blobs?" , "Result of outbreak, possibly" , "Acne" , "Catches sight of" , "Blemishes" .)