Leftover unpalatable Scotch egg, primarily indicating a bad egg (5)
I believe the answer is:
louse
'a bad egg' is the definition.
(both can mean a bad person)
'leftover unpalatable scotch egg primarily' is the wordplay.
'leftover' becomes 'LO' (abbreviations for left and over).
'primarily' says to take the initial letters.
The initial letters of 'unpalatable scotch egg' is 'use'.
'lo'+'use'='LOUSE'
'indicating' is the link.
(Other definitions for louse that I've seen before include "Parasite; nasty person" , "25 [PARASITE]" , "Wingless parasitic insect" , "Insect -- nasty character, undeserving of respect" , "Wingless bloodsucking insect" .)