Bad egg or left-over milk? (5)
I believe the answer is:
louse
'bad egg' is the definition.
(both can mean a bad person)
'left-over milk?' is the wordplay.
'left' becomes 'l' (common abbreviation).
'over' becomes 'o' (cricket abbreviation).
'milk?' becomes 'use' (both can mean to exploit).
'l'+'o'+'use'='LOUSE'
'or' is the link.
(Other definitions for louse that I've seen before include "Wingless bloodsucking insect" , "Human parasite" , "Contemptible type" , "Rogue" , "Wingless parasitic insect" .)