Vitamin A (7)
I believe the answer is:
retinol
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">vitamin a</span>' is the definition.<br/ ><span class="explanation-format__explanation">(vitamin A refers to retinol)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for retinol that I've seen before include "certain vitamin" , "vitamin essential for good eyesight" .)