Pub includes old swine (4)
I believe the answer is:
boar
'swine' is the definition.
(male pig)
'pub includes old' is the wordplay.
'pub' becomes 'bar' (both are drinking establishments).
'includes' means one lot of letters goes inside another.
'old' becomes 'o' (abbreviation).
'bar' placed around 'o' is 'BOAR'.
(Other definitions for boar that I've seen before include "Wild swine" , "A male pig, sounds tedious" , "Uncastrated male pig" , "Wild animal, sounds like drill" , "Resting place for birds or bats" .)