Spoil left overs then finish (5,2)

I believe the answer is:

louse up

'spoil' is the definition.
(synonyms)

'left overs then finish' is the wordplay.
'left' becomes 'l' (common abbreviation).
'overs' becomes 'o' (cricket abbreviation).
'then' means one lot of letters go next to another.
'finish' becomes 'useup' (to use something up is to finish it off).
'l'+'o'+'useup'='LOUSE UP'

(Another definition for louse up that I've seen is " Spoil".)

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