Left remains a scourge (4)
I believe the answer is:
lash
'a scourge' is the definition.
(thesaurus)
'left remains' is the wordplay.
'left' becomes 'l' (common abbreviation).
'remains' becomes 'ash' (I've seen this before).
'l'+'ash'='LASH'
(Other definitions for lash that I've seen before include "Whip or scourge" , "Strike with a whip" , "Birch" , "Beat; fasten with cord" , "Sweep" .)