Concerning a boy's excuses (7)
I believe the answer is:
reasons
'excuses' is the definition.
(I've seen this before)
'concerning a boy's' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'boy's' becomes 'sons' ('son' can be a synonym of 'boy').
're'+'a'+'sons'='REASONS'
(Other definitions for reasons that I've seen before include "Ness oar (anag.)" , "Rational motives" , "Logically deduces" , "Justifications or motives" , "Motives, explanations" .)