Pub contains old swine (4)
I believe the answer is:
boar
'swine' is the definition.
(male pig)
'pub contains old' is the wordplay.
'pub' becomes 'bar' (both are drinking establishments).
'contains' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'bar' placed around 'o' is 'BOAR'.
(Other definitions for boar that I've seen before include "Wild animal, sounds like drill" , "Wild pig sounds tedious" , "Uncastrated male hog" , "Male swine" , "Resting place for birds or bats" .)