Drink's bottomless bottom (3)
I believe the answer is:
rum
'drink's' is the definition.
(rum is a kind of drink)
'bottomless bottom' is the wordplay.
'bottomless' means to remove the last letter (the letter at the bottom is removed).
'bottom' becomes 'rump' (rump can informally mean someone's bottom).
'rump' with its last letter taken off is 'RUM'.
(Other definitions for rum that I've seen before include "Strange; spirit" , "Odd sort of spirit used in grog" , "Droll" , "Odd kind of drink for sailor" , "Odd, like sailor's drink" .)