Quote by setter's flipping sickening (6)
I believe the answer is:
emetic
'sickening' is the definition.
(I've seen this before)
'quote by setter's flipping' is the wordplay.
'quote' becomes 'cite' ('cite' can be a synonym of 'quote').
'by' is a charade indicator (letters next to each other).
'setter' becomes 'me' (the setter of the clue).
'flipping' says the letters should be written in reverse.
'cite'+'me'='citeme'
'citeme' back-to-front is 'EMETIC'.
(Other definitions for emetic that I've seen before include "Cite me as containing sick-making substance" , "It sickens one" , "Ipecacuanha is one" , "Causing to vomit" , "Class including frogs, toads, newts, etc" .)