Annual Henley-on-Thames event (7)
I believe the answer is:
regatta
'annual henley-on-thames event' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for regatta that I've seen before include "races at sea" , "Yacht-racing event" , "for crafty rivals" , "event at the seaside" , "Organised boat races" .)