Counterfeit coin (4)
I believe the answer is:
slug
'counterfeit coin' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for slug that I've seen before include "Wallop" , "Kind of snail without a shell - or a bullet" , "Shell-less mollusc ? bullet" , "Slimy pest" , "Garden mollusc" .)