Briny expanse (3)
I believe the answer is:
sea
'briny expanse' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sea that I've seen before include "Body of saltwater" , "It's stocked with fish" , "North, say" , "Watery expanse" , "The Aegean, for example" .)