Smart guy ringing secretary (7)
I believe the answer is:
capable
'smart' is the definition.
(thesaurus)
'guy ringing secretary' is the wordplay.
'guy' becomes 'cable' (I've seen this in other clues).
'ringing' means one lot of letters goes inside another.
'secretary' becomes 'pa' (I've seen this in other clues).
'cable' going around 'pa' is 'CAPABLE'.
(Other definitions for capable that I've seen before include "Efficient" , "Competent, having a certain ability" , "Able, proficient" , "Of a person, competent" , "Able, competent" .)