Upstart in live leftovers (7)
I believe the answer is:
residue
'leftovers' is the definition.
(I've seen this in another clue)
'upstart in live' is the wordplay.
'upstart' becomes 'u' (start of the word 'up').
'in' indicates putting letters inside.
'live' becomes 'reside' (synonyms).
'u' placed inside 'reside' is 'RESIDUE'.
(Other definitions for residue that I've seen before include "Dire use of the remainder" , "Remaining portion" , "What remains of something" , "Leftovers" , "What is left, maybe dregs" .)