Heavy weapon (4)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
cosh
'heavy weapon' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cosh that I've seen before include "Club used as a weapon" , "Personal weapon" , "From this, blow" , "Sap" , "Blackjack" .)