Pub accepts old swine (4)
I believe the answer is:
boar
'swine' is the definition.
(male pig)
'pub accepts old' is the wordplay.
'pub' becomes 'bar' (both are drinking establishments).
'accepts' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'bar' going around 'o' is 'BOAR'.
(Other definitions for boar that I've seen before include "Wild animal, sounds like drill" , "Uncastrated male hog" , "Sow's mate" , "A male pig, sounds tedious" , "Uncastrated male pig" .)