Comet heralded in part by clear sky (5)
I believe the answer is:
ether
'clear sky' is the definition.
(I've seen this in another clue)
'comet heralded in part' is the wordplay.
'in part' indicates the answer is hidden within the clue.
'ETHER' is hidden in the letters of 'comet heralded'.
'by' is the link.
(Other definitions for ether that I've seen before include "Volatile solvent" , "Imaginary interstellar medium" , "Some were put out by this" , "Light liquid used as anaesthetic" , "Anaesthetic liquid; clear sky" .)