Are worried stiff about the gun (7)

I believe the answer is:

firearm

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'the gun' is the definition.
(firearm is a kind of gun)

'are worried stiff about' is the wordplay.
'worried' indicates an anagram (worry can mean to interfere with or disturb).
'stiff' becomes 'firm' (I can't explain this - if you can you should believe this answer much more).
'about' indicates putting letters inside.
'are' is an anagram of 'ear'.
'ear' put inside 'firm' is 'FIREARM'.

Can you help me to learn more?

(Other definitions for firearm that I've seen before include "Rod" , "dangerous weapon" , "Hand-held weapon" , "Lethal Weapon" , "Colt possibly" .)

I've seen this clue in the Evening Standard.

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