Fish, black when small (4)
I believe the answer is:
bass
'fish' is the definition.
(I've seen this before)
'black when small' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'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 "Fish (which sings?)" , "Sea fish; voice" , "Large fish - singer" , "Low fellow" , "Booming percussion instrument" .)