It could be great at a sailing event! (7)
I believe the answer is:
regatta
'a sailing event' is the definition.
(I've seen this in another clue)
'it could be great at' is the wordplay.
'it could be' indicates anagramming the letters (the same letters could be an anagram).
'great'+'at'='greatat'
'greatat' anagrammed gives 'REGATTA'.
(Other definitions for regatta that I've seen before include "Yacht-racing event" , "Yacht-race event" , "races at sea" , "Sports event" , "Boat festival" .)