Men trapped in river flood (7)
I believe the answer is:
torrent
'flood' is the definition.
(a torrent is a large amount of water)
'men trapped in river' is the wordplay.
'men' becomes 'OR' (military abbreviation for Other Ranks).
'trapped in' is an insertion indicator.
'river' becomes 'trent' (River Trent).
'or' going into 'trent' is 'TORRENT'.
(Other definitions for torrent that I've seen before include "Host" , "fast-moving flow" , "Overwhelming deluge" , "Rush of water" , "Heavy stream" .)