Efficient line about father (7)
I believe the answer is:
capable
'efficient' is the definition.
(I've seen this in another clue)
'line about father' is the wordplay.
'line' becomes 'cable' (cable is a kind of line).
'about' means one lot of letters goes inside another.
'father' becomes 'pa' (pa can mean dad or father).
'cable' going around 'pa' is 'CAPABLE'.
(Other definitions for capable that I've seen before include "Able, competent" , "Able, proficient" , "Of a person, competent" , "Adept" , "qualified" .)