Was a fink (6)
I believe the answer is:
ratted
'was a fink' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ratted that I've seen before include "Deserted the side, rodent-like" , "Deserted; very drunk" , "Broke promise in treacherous or rodent style" , "Betrayed or informed, the rodent" , "Betrayed like a rodent" .)