Half-cut banter in bar (4)
I believe the answer is:
Here is my best explanation:
'bar' is the definition.
(rail is a kind of bar)
'half-cut banter' is the subsidiary indication.
'half-cut' suggests halving the letters.
'banter' becomes 'raillery' ('raillery' can be a synonym of 'banter').
'raillery' halved 'rail'.
'in' is the link.
(Other definitions for rail that I've seen before include "Scold, berate", "British --; picture --", "Bar; waterbird", "The permanent way", "Part of train track", "Track for wheels", "Bar of rolled steel", "Fulminate - wading bird", "Safety bar; bird" and "Abuse, scold".)