Slough service carries soldiers (6)
I believe the answer is:
morass
'slough' is the definition.
(I've seen this before)
'service carries soldiers' is the wordplay.
'service' becomes 'mass' (I've seen this in other clues).
'carries' is an insertion indicator.
'soldiers' becomes 'or' (Other Rank).
'mass' placed around 'or' is 'MORASS'.
(Other definitions for morass that I've seen before include "Slough - marsh" , "Bog or marsh" , "Area of marshy or swampy ground" , "Area of boggy ground" , "Bog; confused situation" .)