Chesapeake Bay is one (7)
I believe the answer is:
estuary
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">chesapeake bay is one</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that Chesapeake Bay is a estuary)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)