Gentleman and unpleasant person capsizing in river (6)
I believe the answer is:
tigris
'river' is the definition.
(Tigris River is an example)
'gentleman and unpleasant person capsizing' is the wordplay.
'gentleman' becomes 'sir' (a man may be addressed as sir).
'and' means one lot of letters go next to another.
'unpleasant person' becomes 'git' (I can't explain this - if you can you should give a lot more credence to this answer).
'capsizing' says the letters should be written backwards.
'sir'+'git'='sirgit'
'sirgit' reversed gives 'TIGRIS'.
'in' is the link.
Can you help me to learn more?
(Other definitions for tigris that I've seen before include "running water" , "Euphrates companion river" , "Iraqi river" , "it runs through Iraq" , "Sit rig into the Asian river" .)