A politician on the left providing illumination (4)
I believe the answer is:
lamp
'illumination' is the definition.
'a politician on the left' is the wordplay.
'politician' becomes 'mp'.
'on' says to put letters next to each other.
'the left' becomes 'l' (common abbreviation).
'a'+'mp'='amp'
'amp' after 'l' is 'LAMP'.
'providing' acts as a link.
(Other definitions for lamp that I've seen before include "This could be standard" , "punch" , "Source of light, usually portable" , "Artificial source of illumination" , "Device providing illumination" .)