Check on board capsized river boat (5)
I believe the answer is:
yacht
'boat' is the definition.
(yacht is a type of boat)
'check on board capsized river' is the wordplay.
'check' becomes 'ch' (abbreviation).
'on board' is an insertion indicator (inserted letters are taken on board).
'capsized' says the letters should be written in reverse (in a down clue, the word turns upside down).
'river' becomes 'tay' (Scottish river).
'tay' reversed gives 'yat'.
'ch' going inside 'yat' is 'YACHT'.
(Other definitions for yacht that I've seen before include "Light sailing-vessel for sport" , "Type of boat" , "Kind of boat" , "Small pleasure boat" , "Light sailing vessel" .)