Seconds for good child ... (5)
I believe the answer is:
sprog
'child' is the definition.
(informal term for a child)
'seconds for good' is the wordplay.
'seconds' becomes 's' (abbreviation).
'for' becomes 'pro' (as in being 'pro' or in favour of something).
'good' becomes 'g' (abbreviation).
's'+'pro'+'g'='SPROG'
(Other definitions for sprog that I've seen before include "Infant" , "Child, colloquially" , "Kid" , "little one" , "Baby (slang)" .)