Say in what way it's a gun (8)
I believe the answer is:
howitzer
'say' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'in what way it's a gun' is the wordplay.
'in what way' becomes 'how' ('how' can be a synonym of 'in what way').
'it's' becomes 'it' (I've seen this in other clues).
'a gun' becomes 'zer' (I can't justify this - if you can you should believe this answer much more).
'how'+'it'+'zer'='HOWITZER'
Can you help me to learn more?
(Other definitions for howitzer that I've seen before include "Gun with a steep angle of fire" , "Shell-firing gun" , "Piece that shoots up" , "Short-range cannon" , "Gun with short barrel used for lobbing shells" .)