Swelling wave (6)
I believe the answer is:
roller
'swelling wave' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for roller that I've seen before include "Sea wave; brilliant bird" , "A large wave or a hair shaper" , "Swelling wave; bird" , "Long heavy sea wave" , "Painting tool" .)