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 "Bird with long legs", "Safety bar; bird", "Scoff", "Waterside bird; part of track", "Bar - marsh bird", "Track - complain - bird", "Bird - track", "Complain about - marsh bird", "Wading bird" and "The permanent way".)