A little bit of sherbet for good kid (5)
I believe the answer is:
sprog
'kid' is the definition.
(I know that sprog is a type of kid)
'a little bit of sherbet for good' is the wordplay.
'a little bit of' indicates taking the first letters (I've seen 'little bits of' mean this).
'for' becomes 'pro' (as in being 'pro' something).
'good' becomes 'g' (abbreviation).
The initial letter of 'sherbet' is 's'.
's'+'pro'+'g'='SPROG'
(Other definitions for sprog that I've seen before include "Baby (slang)" , "Child; recruit (mil. slang)" , "Child, colloquially" , "little one" , "Infant" .)