Mathematical statement butchered in a quote (8)
I believe the answer is:
'mathematical statement' is the definition.
(equation is a kind of mathematical statement)
'butchered in a quote' is the wordplay.
'butchered' is an anagram indicator (I've seen 'butcher' mean this).
'inaquote' with letters rearranged gives 'EQUATION'.
(Other definitions for equation that I've seen before include "Ambrose --, author of The Devil's Dictionary" , "feature of maths" , "Statement that two things have the same value" , "Mathemical statement using = sign" , "Situation where many factors need consideration" .)