Marsh otherwise in service (6)
I believe the answer is:
'marsh' is the definition.
(I've seen this in another clue)
'otherwise in service' is the wordplay.
'otherwise' becomes 'or'.
'in' is an insertion indicator.
'service' becomes 'mass' (kind of church service).
'or' inserted within 'mass' is 'MORASS'.
(Other definitions for morass that I've seen before include "Mire, quagmire" , "mess" , "Area of marshy or swampy ground" , "Confusion" , "Slough - marsh" .)