Settle advance payment before party (7)
I believe the answer is:
subside
'settle' is the definition.
(synonyms)
'advance payment before party' is the wordplay.
'advance payment' becomes 'sub' (subscription payments due to an organisation).
'before' says to put letters next to each other.
'party' becomes 'side' (eg a side or party in a contract).
'sub'+'side'='SUBSIDE'
(Other definitions for subside that I've seen before include "Fall back" , "Wear off or die down" , "Diminish" , "Collapse" , "Cave in" .)