Substance in tea (6)
I believe the answer is:
tannin
'substance in tea' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)