Vitamin C source (4)
I believe the answer is:
lime
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">vitamin c source</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for lime that I've seen before include "Mailed item" , "Bright light green" , "Green fruit -- green colour" , "Citrus tree - non-citrus tree" , "Type of stone" .)