Crowd around other ranks in swamp (6)
I believe the answer is:
morass
'swamp' is the definition.
(I've seen this before)
'crowd around other ranks' is the wordplay.
'crowd' becomes 'mass' (massing is a kind of crowding).
'around' is an insertion indicator.
'other ranks' becomes 'or' (military abbreviation).
'mass' placed around 'or' is 'MORASS'.
'in' acts as a link.
(Other definitions for morass that I've seen before include "Bog or swamp" , "Slough, bog" , "Tract of soft, wet ground" , "Confusion" , "Area of boggy ground" .)