Something that dissolves, once mixed with tea (7)
I believe the answer is:
acetone
'something that dissolves' is the definition.
(chemical used as a solvent)
'once mixed with tea' is the wordplay.
'mixed with' is an anagram indicator.
'once' mixed up with 'tea' can make 'ACETONE'.
(Other definitions for acetone that I've seen before include "Colourless solvent" , "it dissolves" , "chemical is the solution" , "Industrial solvent" , "Common solvent" .)