Flatfish; only (4)
I believe the answer is:
sole
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">flatfish</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(sole is a kind of flatfish)</span><br/ ><br/ >'<span class="explanation-format__original">only</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that only can be written as sole)</span><br/ ><br/ ></span>
(Other definitions for sole that I've seen before include "Kind of fish - lose it" , "Exclusive class of fish" , "piece of leather" , "Underside of the foot" , "A lonely sort of flatfish" .)