Party to start from our pub walls (2,2)
I believe the answer is:
in on
'party to' is the definition.
(I've seen this in another clue)
'start from our pub walls' is the wordplay.
'start from' indicates taking the first letters.
'pub' becomes 'inn' (an inn is a type of pub).
'walls' is an insertion indicator (to 'wall' can mean to contain).
The first letter of 'our' is 'o'.
'o' put into 'inn' is 'IN ON'.
(Other definitions for in on that I've seen before include "Participating in - knowing about" , "Privy to" , "Wise to" , "Aware of secret" , "Sharing with" .)