Neighbours often share this celebration with everyone (5,4)
I believe the answer is:
party wall
'neighbours often share this' is the definition.
(wall shared by two buildings or rooms)
'celebration with everyone' is the wordplay.
'celebration' becomes 'party' (I've seen this before).
'with' becomes 'w' (abbreviation**).
'everyone' becomes 'all' (I've seen this before).
'party'+'w'+'all'='PARTY-WALL'
(Other definitions for party wall that I've seen before include "Divider of joined buildings" , "may be shared between neighbours" , "division in the House" , "That shared by two houses" , "Divider common to two rooms or buildings" .)