Rationale of 'About a Boy' (6)
I believe the answer is:
reason
'rationale' is the definition.
(thesaurus)
'about a boy' is the wordplay.
'about' becomes 'rea' (I've seen this in other clues).
'a boy' becomes 'son' ('son' can be a synonym of 'boy'**).
'rea'+'son'='REASON'
'of' is the link.
(Other definitions for reason that I've seen before include "Motivation or cause" , "Cause to perform" , "Power of thought" , "Justification needed" , "Reach logical conclusions" .)