Estuary language (5)
I believe the answer is:
creek
I believe this is a double definition.
'estuary' is the first definition.
(thesaurus)
'language' is the second definition.
(Creek is an example)
(Other definitions for creek that I've seen before include "Rivulet" , "Small stream in the US" , "Cove" , "Small water course" , "Narrow bay" .)