Arm skimmed dessert (5)
I believe the answer is:
rifle
'arm' is the definition.
(I know that rifle is a type of firearm)
'skimmed dessert' is the wordplay.
'skimmed' means to remove the first letter (to skim something is to take the top off it).
'dessert' becomes 'trifle' (trifle is a kind of dessert).
'trifle' with its first letter taken away is 'RIFLE'.
(Other definitions for rifle that I've seen before include "Author of report" , "Ransack; a weapon" , "Long-barrelled weapon, normal army equipment" , "Gun; sack" , "Ransack - firearm" .)