Competent secretary gets in line (7)
I believe the answer is:
capable
'competent' is the definition.
('capable' can be similar in meaning to 'competent')
'secretary gets in line' is the wordplay.
'secretary' becomes 'pa' (I've seen this in other clues).
'gets in' is an insertion indicator.
'line' becomes 'cable' (cable is a kind of line).
'pa' inserted into 'cable' is 'CAPABLE'.
(Other definitions for capable that I've seen before include "Efficient" , "Adept" , "Of a person, competent" , "Able, proficient" , "Able, competent" .)