Small, effeminate imp! (5)
I believe the answer is:
scamp
'imp' is the definition.
(I know that imp can be written as scamp)
'small effeminate' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'effeminate' becomes 'camp' (I've seen this before).
's'+'camp'='SCAMP'
(Other definitions for scamp that I've seen before include "A young rascal misbehaving in camps" , "little monkey" , "Impish youngster" , "Tearaway" , "Mischief-maker" .)