Swine in love entering pub (4)
I believe the answer is:
boar
'swine' is the definition.
(male pig)
'love entering pub' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'entering' indicates putting letters inside.
'pub' becomes 'bar' (both are drinking establishments).
'o' going inside 'bar' is 'BOAR'.
'in' acts as a link.
(Other definitions for boar that I've seen before include "Wild swine" , "Wild animal, sounds like drill" , "Uncastrated male pig" , "Wild pig sounds tedious" , "Sow's mate" .)