Member meeting unknown host (4)
I believe the answer is:
army
'host' is the definition.
(thesaurus)
'member meeting unknown' is the wordplay.
'member' becomes 'arm' (arm is a kind of member**).
'meeting' is a charade indicator (letters next to each other).
'unknown' becomes 'y' (algebra).
'arm'+'y'='ARMY'
(Other definitions for army that I've seen before include "See 7" , "Land-fighting force" , "A defence force" , "fighters" , "Large number (of ants?)" .)