Party to love cracking pub (2,2)
I believe the answer is:
in on
'party to' is the definition.
(I've seen this in another clue)
'love cracking pub' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'cracking' means one lot of letters goes inside another (inserted letters go into a gap or crack).
'pub' becomes 'inn' (an inn is a type of pub).
'o' placed inside 'inn' is 'IN ON'.
(Other definitions for in on that I've seen before include "Aware of secret" , "Aware of - being part of" , "Sharing with" , "Wise to" , "Participating in - knowing about" .)