Slough provides 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' indicates putting letters inside.
'service' becomes 'mass' (I've seen this in other clues).
'or' placed within 'mass' is 'MORASS'.
'provides' is the link.
(Other definitions for morass that I've seen before include "Bog or marsh" , "Muddled situation" , "Mire, quagmire" , "mess" , "Tract of soft, wet ground" .)