Clever old man brought into line (7)
I believe the answer is:
capable
'clever' is the definition.
(thesaurus)
'old man brought into line' is the wordplay.
'old man' becomes 'pa' (both can mean father).
'brought into' is an insertion indicator.
'line' becomes 'cable' (cable is a kind of line).
'pa' inserted inside 'cable' is 'CAPABLE'.
(Other definitions for capable that I've seen before include "Competent, having a certain ability" , "'Clever, able (7)'" , "Able, proficient" , "Of a person, competent" , "qualified" .)