Will get out a gun in retaliation - that's the snag (8)
I believe the answer is:
drawback
'the snag' is the definition.
(I've seen this before)
'will get out a gun in retaliation' is the wordplay.
'will' says to put letters next to each other (I've seen this in other clues).
'get out' becomes 'draw' (synonyms).
'a gun in retaliation' becomes 'back' (I am not sure about this - if you are sure you should believe this answer much more).
'draw' put next to 'back' is 'DRAWBACK'.
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for drawback that I've seen before include "Limitation, disadvantage" , "Catch" , "Snag" , "Disadvantage, problem" , "Hindrance - disadvantage" .)