French marshal backed by party’s senior member (5)
I believe the answer is:
'senior member' is the definition.
(I've seen this before)
'french marshal backed by party's' is the wordplay.
'french marshal' becomes 'ney' (I've seen this in other clues).
'backed' says the letters should be written in reverse.
'by' says to put letters next to each other.
'party' becomes 'do' (do is a kind of party).
'ney' back-to-front is 'yen'.
'yen' after 'do' is 'DOYEN'.
(Other definitions for doyen that I've seen before include "Dean of an academy" , "Most prominent expert" , "Top expert in a field" , "As senior statesman" , "Senior member of a professional group" .)