Left the thing to be illuminated (3)
I believe the answer is:
lit
'be illuminated' is the definition.
('lit' can be a synonym of 'illuminated')
'left the thing' is the wordplay.
'left' becomes 'l' (common abbreviation).
'the thing' becomes 'it'.
'l'+'it'='LIT'
'to' is the link.
(Other definitions for lit that I've seen before include "Set alight" , "landed here" , "Set fire to" , "Burning" , "Ignited, say cigarette" .)