Weak sun has radiance (6)
I believe the answer is:
slight
'weak' is the definition.
('slight' can be similar in meaning to 'weak')
'sun has radiance' is the wordplay.
'sun' becomes 's'.
'has' says to put letters next to each other.
'radiance' becomes 'light' (I've seen this before).
's'+'light'='SLIGHT'
(Other definitions for slight that I've seen before include "Small in degree; insult" , "Snub - not much!" , "Insult - not important!" , "Put down" , "Deliberate discourteous act" .)