Man is one (4)
I believe the answer is:
isle
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">man is one</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 isle that I've seen before include "eg Man, Ely" , "Land amid water" , "Small area of ground" , "Lies (anag)" , "Small area of land in water" .)