Yellow element (7)
I believe the answer is:
sulphur
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">yellow element</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for sulphur that I've seen before include "S (chem.)" , "Element, atomic no. 16" , "Chemical used in match heads" , "Highly flammable yellow element" , "Chemical element" .)