Other people crossing river really capsized vessel (7)
I believe the answer is:
thermos
'vessel' is the definition.
(thermos is a kind of vessel)
'other people crossing river really capsized' is the wordplay.
'other people' becomes 'them'.
'crossing' is an insertion indicator.
'river' becomes 'r'.
'really' becomes 'so' (both can mean 'extremely').
'capsized' shows that the letters should be reversed in order.
'so' written backwards gives 'os'.
'them' going around 'r' is 'therm'.
'therm'+'os'='THERMOS'
(Other definitions for thermos that I've seen before include "Heat-retaining flask for mothers" , "Container to retain heat" , "Flask trade mark" , "Vacuum flask for mothers" .)