Unctuous sergeant major meeting host (6)
I believe the answer is:
smarmy
'unctuous' is the definition.
(I know that unctuous can be written as smarmy)
'sergeant major meeting host' is the wordplay.
'sergeant major' becomes 'SM' (abbreviation for sergeant major).
'meeting' says to put letters next to each other.
'host' becomes 'army' (both can mean a large number of people).
'sm'+'army'='SMARMY'
(Other definitions for smarmy that I've seen before include "Oily" , "Excessively flattering (R, not L)" , "Smooth" , "Unctuous" , "Oleaginous" .)