Adamant about key American right to have a gun (7)
I believe the answer is:
firearm
'a gun' is the definition.
(I know that gun can be written as firearm)
'adamant about key american right' is the wordplay.
'adamant' becomes 'firm'.
'about' means one lot of letters goes inside another.
'key' becomes 'e' (musical key).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'right' becomes 'r' (common abbreviation).
'e'+'a'+'r'='ear'
'firm' enclosing 'ear' is 'FIREARM'.
'to have' is the link.
(Other definitions for firearm that I've seen before include "Lethal Weapon" , "Hand-held weapon" , "Rod" , "piece" , "Pistol, rifle or gun" .)