Substance in tea (6)
I believe the answer is:
tannin
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">substance in tea</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 tannin that I've seen before include "Plant substance used in, eg, leather-making, medicine and red wine" , "gives red wine its distinctive flavour" , "Bitter substance in wine" , "Bitter substance in red wines" , "Substance used in treating leather" .)