Obsequious troops support sergeant major
I believe the answer is:
smarmy
'obsequious' is the definition.
(I've seen this before)
'troops support sergeant major' is the wordplay.
'troops' becomes 'army' (I've seen this in another clue).
'support' means one lot of letters go next to another (in a down clue, some letters hold others up).
'sergeant major' becomes 'SM' (abbreviation for sergeant major).
'army' put after 'sm' is 'SMARMY'.
(Other definitions for smarmy that I've seen before include "Oleaginous" , "Conceal oneself" , "Oily" , "Fawning" , "Slick" .)