A once-rough sea (5)
I believe the answer is:
ocean
'sea' is the definition.
(synonyms)
'a once-rough' is the wordplay.
'rough' is an anagram indicator (a rough or broken ordering of the letters).
'a'+'once'='aonce'
'aonce' with letters rearranged gives 'OCEAN'.
(Other definitions for ocean that I've seen before include "Sea - canoe (anag)" , "eg Indian, Atlantic" , "Huge body of water" , "Vast sea" , "Huge expanse" .)