Concerning a boy competent and sensible (10)
I believe the answer is:
reasonable
'sensible' is the definition.
(I know that sensible can be written as reasonable)
'concerning a boy competent' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'boy' becomes 'son' (synonyms).
'competent' becomes 'able' (similar in meaning).
're'+'a'+'son'+'able'='REASONABLE'
'and' acts as a link.
(Other definitions for reasonable that I've seen before include "Fair, rational" , "Fair - inexpensive" , "Not excessive or extreme" , "Sound" , "description of rationality" .)