Bright light upsets these Greek characters (3)
I believe the answer is:
nus
'these greek characters' is the definition.
(nu is a letter of the Greek alphabet)
'bright light upsets' is the wordplay.
'bright light' becomes 'sun' (I've seen this in another clue).
'upsets' says the letters should be written backwards.
'sun' backwards is 'NUS'.
(Other definitions for nus that I've seen before include "Greek letters, N" , "Letters from abroad" , "students" .)