Are strangely adamant about the weapon (7)
I believe the answer is:
'the weapon' is the definition.
(firearm is a kind of weapon)
'are strangely adamant about' is the wordplay.
'strangely' is an anagram indicator.
'adamant' becomes 'firm' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'about' indicates putting letters inside.
'are' anagrammed gives 'ear'.
'ear' going into 'firm' is 'FIREARM'.
Can you help me to learn more?
(Other definitions for firearm that I've seen before include "One's fired" , "piece" , "Hand-held weapon" , "Rod" , "Colt possibly" .)