Some critic and leading light (6)
I believe the answer is:
candle
'light' is the definition.
(candle is a kind of light)
'some critic and leading' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'CANDLE' can be found hidden inside 'critic and leading'.
(Other definitions for candle that I've seen before include "It's better to light one than curse the darkness, it's said" , "Artificial light" , "something wicked" , "Wax cylinder" , "Old form of illumination" .)