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)" .)

I've seen this clue in The Sun, The Telegraph and The Times.

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