Slightly less illumination can be clever (3)
I believe the answer is:
sly
'be clever' is the definition.
(thesaurus)
'slightly less illumination' is the wordplay.
'less' is a deletion indicator.
'illumination' becomes 'light' (light is a kind of illumination).
'slightly' with 'light' removed is 'SLY'.
'can' acts as a link.
(Other definitions for sly that I've seen before include "Cunning, foxy" , "Devious and furtive" , "Artful" , "Doing things in unpleasant and secret way" , "'Cunning, wily (3)'" .)