Quote me retrospectively? It'll make you sick (6)
I believe the answer is:
'it'll make you sick' is the definition.
'quote me retrospectively?' is the wordplay.
'quote' becomes 'cite' ('cite' can be a synonym of 'quote').
'retrospectively?' shows that the letters should be reversed in order (I've seen 'retrospectively' mean this).
'citeme' back-to-front is 'EMETIC'.
(Other definitions for emetic that I've seen before include "Medicine to make you sick" , "Class including frogs, toads, newts, etc" , "Cite me as containing sick-making substance" , "medicament" , "Sickening" .)