Four regulars in pub having a joke (2,3)
I believe the answer is:
in fun
'having a joke' is the definition.
'four regulars in pub' is the wordplay.
'regulars' means one should take alternating letters.
'in' indicates putting letters inside.
'pub' becomes 'inn' (I've seen this before).
The alternating letters of 'four' are 'fu'.
'fu' placed within 'inn' is 'IN FUN'.
(Other definitions for in fun that I've seen before include "to raise a smile?" , "for a laugh" , "Not intended seriously" , "As a joke - not seriously" .)