Think about a boy (6)
I believe the answer is:
reason
'think' is the definition.
(I know that reason is a kind of the think)
'about a boy' is the wordplay.
'about' becomes 'rea' (I've seen this in other clues).
'a boy' becomes 'son' (synonyms).
'rea'+'son'='REASON'
(Other definitions for reason that I've seen before include "brain operations" , "Sanity of senora" , "'Motivation, grounds (6)'" , "Sanity and logic" , "Purpose" .)