Unusually square but short old gun (8)
I believe the answer is:
arquebus
'old gun' is the definition.
'arquebus' can be an answer for 'gun' (arquebus is a kind of gun). I'm unsure of the 'old' bit.
'unusually square but short' is the wordplay.
'unusually' indicates anagramming the letters.
'short' means to remove the last letter.
'square'+'but'='squarebut'
'squarebut' with its final letter taken away is 'squarebu'.
'squarebu' anagrammed gives 'ARQUEBUS'.
Can you help me to learn more?
(Other definitions for arquebus that I've seen before include "arm" , "Gun called a hagbut" .)