Oily sump regularly scrubbed by troops (6)
I believe the answer is:
smarmy
'oily' is the definition.
(I know that oily can be written as smarmy)
'sump regularly scrubbed by troops' is the wordplay.
'regularly scrubbed' indicates alternate letters (letters removed at regular intervals).
'by' says to put letters next to each other.
'troops' becomes 'army' (I've seen this in another clue).
The alternating letters of 'sump' are 'sm'.
'sm'+'army'='SMARMY'
(Other definitions for smarmy that I've seen before include "Excessively ingratiating" , "Conceal oneself" , "Smooth" , "Unctuous" , "Bootlicking" .)