Poet scratching his bottom in pub (3)
I believe the answer is:
bar
'in pub' is the definition.
(I know that bar has the is part of gin mill)
'poet scratching his bottom' is the wordplay.
'poet' becomes 'bard' (bard is a kind of poet).
'scratching his bottom' means to remove the last letter (the bottom letter is scratched or ignored).
'bard' with its last letter taken away is 'BAR'.
(Other definitions for bar that I've seen before include "Barrier; pressure unit" , "Exclude; sandbank" , "Sandbank; barrier" , "sovereign" , "Rigid rod" .)