Horse rears, carrying glutton making witty remark (7)
I believe the answer is:
'witty remark' is the definition.
(I've seen this before)
'horse rears carrying glutton' is the wordplay.
'horse' becomes 'mare' (mare is a kind of horse).
'rears' is a reversal indicator (I've seen 'rearing' mean this (as in a horse rearing up)).
'carrying' is an insertion indicator.
'glutton' becomes 'pig' (I've seen this in another clue).
'mare' backwards is 'eram'.
'eram' placed around 'pig' is 'EPIGRAM'.
'making' is the link.
(Other definitions for epigram that I've seen before include "Witty, paradoxical remark" , "Clever remark" , "Brief witty saying" , "Gnome" , "Form of wit" .)