Clad in jacket, a low-down participant in espionage (6,5)
I believe the answer is:
double agent
'participant in espionage' is the definition.
(a double agent performs espionage)
'clad in jacket a low-down' is the wordplay.
'clad in' is an insertion indicator.
'jacket' becomes 'doublet' (doublet is a kind of jacket).
'low down' becomes 'gen' (both can mean information).
'a'+'gen'='agen'
'doublet' enclosing 'agen' is 'DOUBLE AGENT'.
(Other definitions for double agent that I've seen before include "AA man?" , "Treacherous spy" .)