Watercourse where nearly everybody follows the vessel (5)
I believe the answer is:
canal
'watercourse' is the definition.
(I know that canal is a type of watercourse)
'nearly everybody follows the vessel' is the wordplay.
'nearly' means to remove the last letter.
'everybody' becomes 'all' (all people).
'follows' is a charade indicator (letters next to each other).
'the vessel' becomes 'can' (type of container).
'all' with its final letter taken away is 'al'.
'al' put after 'can' is 'CANAL'.
'where' acts as a link.
(Other definitions for canal that I've seen before include "Passage" , "Man-made waterway" , "Inland waterway" , "Man-made channel of water" , "Grand Union perhaps" .)