Chesapeake Bay is one (7)
I believe the answer is:
estuary
'chesapeake bay is one' is the definition.
(I know that Chesapeake Bay is a estuary)
This is all the clue.
(Other definitions for estuary that I've seen before include "Bristol Channel?" , "River outlet" , "Wide river mouth" , "Here one comes close to the main" , "Where tide meets stream" .)