Watery expanse
I believe the answer is:
sea
'watery expanse' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sea that I've seen before include "E.g Aral or Weddell" , "All at . . . - totally confused" , "Body of saltwater" , "The briny" , "Expanse of salt water" .)