One of the bright lights in libertine ontology (4)
I believe the answer is:
neon
'one of the bright lights' is the definition.
'in libertine ontology' is the wordplay.
'in' says the answer is hidden in the clue.
'NEON' is hidden in the letters of 'libertine ontology'.
(Other definitions for neon that I've seen before include "One of the six inert gasses" , "One of the six inert gases" , "Gas used in fluorescent lights" , "Gas used in fluorescent lamps" , "Gas - light" .)