Lump soldiers pulled out of swamp (4)
I believe the answer is:
mass
'lump' is the definition.
(I've seen this in another clue)
'soldiers pulled out of swamp' is the wordplay.
'soldiers' becomes 'OR' (Other Ranks).
'pulled out of' indicates named letters should be taken away.
'swamp' becomes 'morass' (I've seen this before).
'morass' with 'or' taken out is 'MASS'.
(Other definitions for mass that I've seen before include "Lump of material" , "Lots of people" , "Great deal" , "seasonal midnight service?" , "Pound, maybe" .)