Defector has a go at undermining company (8)
I believe the answer is:
turncoat
'defector' is the definition.
(turncoat is a kind of defector)
'a go at undermining company' is the wordplay.
'a go' becomes 'turn' (as in one's go or turn in a game).
'undermining' is a charade indicator (letters next to each other).
'company' becomes 'co' (abbreviation).
'at' after 'co' is 'coat'.
'turn'+'coat'='TURNCOAT'
'has' is the link.
(Other definitions for turncoat that I've seen before include "Traitor, defector" , "Judas" , "Actor nut may be a traitor" , "Traitor with inverted garment" , "Traitor with reversed garment" .)