Senior member’s passionate desire to support party (5)
I believe the answer is:
doyen
'senior member's' is the definition.
(a senior or respected figure)
'passionate desire to support party' is the wordplay.
'passionate desire' becomes 'yen' (yen can mean a craving for something).
'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' put after 'do' is 'DOYEN'.
(Other definitions for doyen that I've seen before include "The oldest of the members" , "Senior male figure in a profession" , "The most respected among experts" , "Dean of an academy" , "Top expert in a field" .)