Large wavy areas (4)
I believe the answer is:
seas
'large wavy areas' is the definition.
Although both the answer and definition are plural nouns, I cannot understand how one could define the other.
This is all the clue.
Can you help me to learn more?
(Other definitions for seas that I've seen before include "Oceans" , "Red and Dead" , "Bodies of salt water" , "Red and Black are" , "Aegean and Mediterranean, say" .)