Unctuous sergeant-major with soldiers (6)
I believe the answer is:
smarmy
'unctuous' is the definition.
(I know that unctuous can be written as smarmy)
'sergeant-major with soldiers' is the wordplay.
'sergeant major' becomes 'SM' (abbreviation for sergeant major).
'with' says to put letters next to each other.
'soldiers' becomes 'army' (group of soldiers).
'sm'+'army'='SMARMY'
(Other definitions for smarmy that I've seen before include "Unctuous" , "Smooth" , "Obsequious" , "Excessively ingratiating" , "Oleaginous" .)