Fish, note, when small (4)
I believe the answer is:
bass
'fish' is the definition.
(I've seen this before)
'note when small' is the wordplay.
'note' becomes 'b' (musical note).
'when' becomes 'as'.
'small' becomes 's' (abbreviation - e.g. clothes size).
'b'+'as'+'s'='BASS'
(Other definitions for bass that I've seen before include "Freshwater fish and lowest voice" , "Performer" , "Low singing voice" , "bottom of sound?" , "Big percussion instrument" .)