Mention being upset by letter that brings everything up (6)
I believe the answer is:
emetic
'mention being upset by letter that brings everything up' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'mention being upset by letter' is the wordplay.
'mention' becomes 'cite' ('cite' can be a synonym of 'mention').
'being upset' says the letters should be written backwards.
'by' is a charade indicator (letters next to each other).
'letter' becomes 'em' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'cite' in reverse letter order is 'etic'.
'etic' after 'em' is 'EMETIC'.
Can you help me to learn more?
(Other definitions for emetic that I've seen before include "Vomiting inducer" , "Medicine to make someone throw up" , "making you throw up" , "it will make one sick" , "Substance given to make one vomit" .)