Child finally goes for good (5)
I believe the answer is:
sprog
'child' is the definition.
(informal term for a child)
'finally goes for good' is the wordplay.
'finally' says to take the final letters.
'for' becomes 'pro' (as in being 'pro' or in favour of something).
'good' becomes 'g' (abbreviation).
The final letter of 'goes' is 's'.
's'+'pro'+'g'='SPROG'
(Other definitions for sprog that I've seen before include "little one" , "Child; recruit (mil. slang)" , "Child, colloquially" , "Infant" , "Kid" .)