Senior member's wish to support party (5)
I believe the answer is:
doyen
'senior member's' is the definition.
(a senior or respected figure)
'wish to support party' is the wordplay.
'wish' becomes 'yen' (I've seen this in another clue).
'to support' is a charade indicator (letters next to each other) (in a down clue, some letters hold others up).
'party' becomes 'do' (do is a kind of party).
'yen' after 'do' is 'DOYEN'.
(Other definitions for doyen that I've seen before include "Greatly respected person" , "The oldest of the members" , "Senior member of a profession or academy" , "As senior statesman" , "Senior male member of a group" .)