Weapon used by erratic flier (5)
I believe the answer is:
rifle
'weapon' is the definition.
(rifle is a kind of weapon)
'erratic flier' is the wordplay.
'erratic' indicates an anagram.
'flier' anagrammed gives 'RIFLE'.
'used by' acts as a link.
(Other definitions for rifle that I've seen before include "lock, stock and barrel" , "disarray" , "Long-barrelled gun" , "Search and rob for army weapon" , "Pillage; a weapon" .)