Argue about a child (6)
I believe the answer is:
reason
'argue' is the definition.
(synonyms)
'about a child' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'child' becomes 'son' (male child).
're'+'a'+'son'='REASON'
(Other definitions for reason that I've seen before include "Reach logical conclusions" , "The right exercise of the mind" , "'Motivation, grounds (6)'" , "Rational motive" , "Justification; sanity" .)