Obsequious host backs officer (6)
I believe the answer is:
smarmy
'obsequious' is the definition.
(I've seen this before)
'host backs officer' is the wordplay.
'host backs' becomes 'sm' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'officer' becomes 'army' (army officer is a kind of officer).
'sm'+'army'='SMARMY'
Can you help me to learn more?
(Other definitions for smarmy that I've seen before include "Unctuous" , "Oleaginous" , "Smooth" , "Unpleasantly suave or ingratiating" , "Oily" .)