Sergeant major facing soldiers? One's unlikely to be this! (6)
I believe the answer is:
smarmy
'one's unlikely to be this' is the definition.
The definition suggests a singular noun but the answer is not.
'sergeant major facing soldiers?' is the wordplay.
'sergeant major' becomes 'SM' (abbreviation for sergeant major).
'facing' is a charade indicator (letters next to each other).
'soldiers?' becomes 'army' (group of soldiers).
'sm'+'army'='SMARMY'
(Other definitions for smarmy that I've seen before include "Oleaginous" , "Obsequious" , "Smooth" , "Bootlicking" , "Fawning" .)