Horse swallows mixed grain and fat (9)
I believe the answer is:
margarine
'fat' is the definition.
(I've seen this before)
'horse swallows mixed grain' is the wordplay.
'horse' becomes 'mare' (mare is a kind of horse).
'swallows' is an insertion indicator.
'mixed' indicates an anagram.
'grain' with letters rearranged gives 'garin'.
'mare' enclosing 'garin' is 'MARGARINE'.
'and' acts as a link.
(Other definitions for margarine that I've seen before include "Reg and Marian make a butter substitute" , "Item of food" , "Fat-based butter substitute" , "Substitute spread for butter" , "Is Ma angrier at this spread?" .)