Supposing it's the twit with the gun I'm to tie up (8)
I believe the answer is:
assuming
'supposing' is the definition.
(I've seen this in another clue)
'the twit with the gun i'm to tie up' is the wordplay.
I cannot really see how this works, but
'twit' could be 'ass' (I've seen this in another clue) and 'ass' is present in the answer.
'i'm' could be 'i' (I've seen this in other clues) and 'i' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'it's' is the link.
Can you help me to learn more?
(Other definitions for assuming that I've seen before include "Taking on - arrogant" , "Taking as true" , "Accepting something without proof" , "Official language of Pakistan" , "Presumptuous, arrogant" .)