Source of shots being set up in part of pub (4)
I believe the answer is:
snug
'part of pub' is the definition.
(a snug is a small, comfortable room in a pub)
'source of shots being set up' is the wordplay.
'source of' suggests taking the first letters.
'being' becomes 'gun' (I can't explain this - if you can you should give a lot more credence to this answer).
'set up' shows that the letters should be reversed in order (in a down clue, letters go up).
The first letter of 'shots' is 's'.
'gun' backwards is 'nug'.
's'+'nug'='SNUG'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for snug that I've seen before include "Well protected or concealed" , "Comfortable -- small secluded room" , "Warm and cosy" , "Close fitting but comfortable" , "Small bar in pub - warm and comfortable" .)