Shilling for good kid (5)
I believe the answer is:
sprog
'kid' is the definition.
(I know that sprog is a type of kid)
'shilling for good' is the wordplay.
'shilling' becomes 's' (abbreviation e.g. Lsd - pounds, shillings, pence).
'for' becomes 'pro' (as in being 'pro' something).
'good' becomes 'g' (abbreviation).
's'+'pro'+'g'='SPROG'
(Other definitions for sprog that I've seen before include "Baby (slang)" , "little one" , "Infant" , "Child, colloquially" , "Kid" .)