A heavenly body has to arrive on time (5)
I believe the answer is:
'a heavenly body' is the definition.
(comet is a kind of heavenly body)
'to arrive on time' is the wordplay.
'to arrive' becomes 'come' (coming is a kind of arriving).
'on' says to put letters next to each other.
'time' becomes 't' (abbreviation).
'has' is the link.
(Other definitions for comet that I've seen before include "Shooting star" , "high-flier" , "Heavenly visitor" , "Tailed space traveller" , "Small extraterrestrial body" .)