Heavy staff used as a weapon (6)
I believe the answer is:
cudgel
'heavy staff used as a weapon' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cudgel that I've seen before include "Short thick stick used as weapon" , "Truncheon-like weapon" , "brain" , "Club used for hitting people" , "Short stout stick" .)