The weapon business, are involved in (7)
I believe the answer is:
firearm
'the weapon' is the definition.
(firearm is a kind of weapon)
'business are involved in' is the wordplay.
'business' becomes 'firm' (firm is a kind of business**).
'involved' is an anagram indicator (involved can mean confusing or complex).
'in' indicates putting letters inside.
'are' is an anagram of 'ear'.
'firm' enclosing 'ear' is 'FIREARM'.
(Other definitions for firearm that I've seen before include "Hand-held weapon" , "One's fired" , "Lethal Weapon" , "Colt possibly" , "Rod" .)