Spy bumping off a fellow (4)
I believe the answer is:
gent
'fellow' is the definition.
('gent' can be a synonym of 'fellow')
'spy bumping off a' is the wordplay.
'spy' becomes 'agent' (I've seen this before).
'bumping off' is a deletion indicator.
'agent' with 'a' taken out is 'GENT'.
(Other definitions for gent that I've seen before include "cove" , "Informal and short man" , "bloke" , "Toff" , "Auctioneer's tool" .)