Adept secretary engaged by Telegraph (7)
I believe the answer is:
capable
'adept' is the definition.
(I've seen this before)
'secretary engaged by telegraph' is the wordplay.
'secretary' becomes 'PA' (abbreviation for personal assistant).
'engaged by' indicates putting letters inside (I've seen this in other clues).
'telegraph' becomes 'cable' (synonyms).
'pa' placed within 'cable' is 'CAPABLE'.
(Other definitions for capable that I've seen before include "Of a person, competent" , "accomplished" , "Competent, having a certain ability" , "Adept" , "'Clever, able (7)'" .)