Slough needs men in service (6)
I believe the answer is:
morass
'slough' is the definition.
(I've seen this before)
'men in service' is the wordplay.
'men' becomes 'or' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'service' becomes 'mass' (I've seen this in other clues).
'or' going into 'mass' is 'MORASS'.
'needs' is the link.
(Other definitions for morass that I've seen before include "Bog or swamp" , "Area of marshy or swampy ground" , "Tract of swampy land" , "Swampy, low-lying land" , "Slough - marsh" .)