Proceed quickly with opening of gin bar (4)
I believe the answer is:
rung
'bar' is the definition.
'proceed quickly with opening of gin' is the wordplay.
'proceed quickly' becomes 'run' (I've seen this before).
'with' means one lot of letters go next to another.
'opening of' suggests taking the first letters.
The initial letter of 'gin' is 'g'.
'run'+'g'='RUNG'
(Other definitions for rung that I've seen before include "Phoned" , "Horizontal bar in ladder" , "Tolled - step on ladder" , "Level in a hierarchy" , "Telephoned" .)