Coming soon, time when the earth cools (5)
I believe the answer is:
night
'when the earth cools' is the definition.
(typically colder part of the day)
'coming soon time' is the wordplay.
'coming soon' becomes 'nigh'.
'time' becomes 't' (abbreviation).
'nigh'+'t'='NIGHT'
(Other definitions for night that I've seen before include "What proverbially follows day" , "period of gloom" , "follow-up to 16 [TWILIGHT]" , "A Hard Day's (the Beatles)" , "Ignorance" .)