Comet part (4)
I believe the answer is:
tail
'comet part' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tail that I've seen before include "Flexible part that extends from an animal's rear" , "Last, rear, part" , "Trail (behind comet)" , "Back end; follow secretly" , "Follow - train" .)