Last imbecile (4)
I believe the answer is:
goon
This clue is a double definition.
'last' is the first definition.
(go on)
'imbecile' is the second definition.
(Other definitions for goon that I've seen before include "He's very stupid" , "Is this show" , "person making fool of himself" , "Hired thug" , "Sellers for example" .)