In lab reheat revolting brew (6,3)
I believe the answer is:
herbal tea
'in' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'lab reheat revolting brew' is the wordplay.
'reheat' becomes 'reh' (I can't justify this - if you can you should believe this answer much more).
'revolting' says the letters should be written backwards.
'brew' becomes 'tea' (I've seen this in another clue).
'lab'+'reh'='labreh'
'labreh' backwards is 'herbal'.
'herbal'+'tea'='HERBAL TEA'
(Other definitions for herbal tea that I've seen before include "liquid concoction" , "Plant infusion" , "healthy drink" , "hot drink" .)