Act on desire to find most senior member (5)
I believe the answer is:
doyen
'most senior member' is the definition.
'act on desire' is the wordplay.
'act on' becomes 'do'.
'desire' becomes 'yen' (yen is a kind of desire).
'do'+'yen'='DOYEN'
'to find' is the link.
(Other definitions for doyen that I've seen before include "As senior statesman" , "Senior member of a professional group" , "Top expert in a field" , "Respected senior member" , "Respected expert" .)