I reused horrible leftovers (7)
I believe the answer is:
residue
'leftovers' is the definition.
(I've seen this before)
'i reused horrible' is the wordplay.
'horrible' is an anagram indicator.
'i'+'reused'='ireused'
'ireused' with letters rearranged gives 'RESIDUE'.
(Other definitions for residue that I've seen before include "Stuff left over" , "Matter left behind after other parts have been taken away" , "rest" , "What's left" , "Remaining portion" .)