Had dropped everything into the marsh (6)
I believe the answer is:
fallen
'had dropped' is the definition.
I can't tell whether this definition defines the answer.
'everything into the marsh' is the wordplay.
'everything' becomes 'all' (all things).
'into' means one lot of letters goes inside another.
'the marsh' becomes 'fen' (synonyms).
'all' going into 'fen' is 'FALLEN'.
Can you help me to learn more?
(Other definitions for fallen that I've seen before include "Tumbled" , "Having dropped by the force of gravity" , "... _____!" (II Samuel)" , "dead" , "ruined" .)