Search messily for a firearm (5)
I believe the answer is:
rifle
This clue is a double definition.
'search messily' is the first definition.
(I've seen this in another clue)
'firearm' is the second definition.
(rifle is a kind of firearm)
'for a' acts as a link.
(Other definitions for rifle that I've seen before include "Rummage" , "Go through in search" , "Gun; thoroughly search" , "shoot?" , "lock, stock and barrel" .)