Casual clothing left in vessel on river (10)
I believe the answer is:
sportswear
'casual clothing' is the definition.
'sportswear' can be an answer for 'clothing' (I've seen this before). I'm not certain of the 'casual' bit.
'left in vessel on river' is the wordplay.
'left in vessel' becomes 'sports' (I can't justify this - if you can you should believe this answer much more).
'on' is a charade indicator (letters next to each other).
'river' becomes 'Wear' (river in the north east of England).
'sports'+'wear'='SPORTSWEAR'
Can you help me to learn more?
(Other definitions for sportswear that I've seen before include "Gear" , "informal suits" , "Casual clothes" , "football kit?" , "in which triathletes swim" .)