Big number, including other ranks, in swampy area (6)
I believe the answer is:
morass
'swampy area' is the definition.
(swampy or boggy area)
'big number including other ranks' is the wordplay.
'big number' becomes 'mass'.
'including' is an insertion indicator.
'other ranks' becomes 'or' (military abbreviation).
'mass' going around 'or' is 'MORASS'.
'in' acts as a link.
(Other definitions for morass that I've seen before include "Bog - situation hard to get out of" , "Slough - marsh" , "dangerous situation" , "Swampy, low-lying land" , "Confusion" .)