Large deep bay (4)
I believe the answer is:
gulf
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">large deep bay</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for gulf that I've seen before include "Deep inlet of sea; chasm" , "Chasm, large gap" , "Wide separation" , "Unbridgeable disparity" , "Wide gap; arm of sea" .)