Competent assistant cuts wire (7)
I believe the answer is:
capable
'competent' is the definition.
(synonyms)
'assistant cuts wire' is the wordplay.
'assistant' becomes 'PA' (personal assistant).
'cuts' is an insertion indicator (some letters cut their way into other letters).
'wire' becomes 'cable' (cable is a kind of wire).
'pa' put within 'cable' is 'CAPABLE'.
(Other definitions for capable that I've seen before include "Adept" , "Competent, having a certain ability" , "accomplished" , "'Clever, able (7)'" , "Able, competent" .)