Mention about upsetting kind of acid (6)
I believe the answer is:
acetic
'kind of acid' is the definition.
(I've seen this in another clue)
'mention about upsetting' is the wordplay.
'mention' becomes 'cite' (to cite is to mention or quote).
'about' becomes 'ca' (circa).
'upsetting' is a reversal indicator.
'cite'+'ca'='citeca'
'citeca' back-to-front is 'ACETIC'.
(Other definitions for acetic that I've seen before include "really sour" , "The acid in vingear" , "in relation to acid" , "Like vinegar's acid" , "Type of acid derived from vinegar" .)