Who incorrectly said it's a gun? (8)
I believe the answer is:
howitzer
'it's a gun?' is the definition.
(howitzer is a kind of gun)
'who incorrectly said' is the wordplay.
'incorrectly' indicates anagramming the letters (I've seen this in other clues).
'said' becomes 'itzer' (I can't justify this - if you can you should give a lot more credence to this answer).
'who' anagrammed gives 'how'.
'how'+'itzer'='HOWITZER'
Can you help me to learn more?
(Other definitions for howitzer that I've seen before include "Gun for shelling" , "Piece that shoots up" , "Shell-firing gun" , "Gun firing shells at a steep angle" , "Short, heavy gun fired at an angle" .)