Sailing competition (8,3)
'sailing competition' 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 "event held on water" , "Boat-race meeting" , "Yacht-race event" , "Rowing, sailing, gala" , "Yachting festival" .)
(Other definitions for americas cup that I've seen before include "sailors want to win it" , "Yacht-racing trophy" , "Yachting prize" , "International yachting trophy" , "main competition" .)