Heavy wave sweeping over vessel in fairground ride (13)
I believe the answer is:
rollercoaster
'fairground ride' is the definition.
(type of fairground attraction)
'heavy wave sweeping over vessel' is the wordplay.
'heavy wave sweeping' becomes 'roller' (roller can mean a large wave).
'over' means one lot of letters go next to another.
'vessel' becomes 'coaster' (I've seen this before).
'roller'+'coaster'='ROLLERCOASTER'
'in' acts as a link.
(Other definitions for rollercoaster that I've seen before include "fairground attraction" , "Big dipper" , "Fairground ride" .)