21 across
Close
temperature
when
sun
goes
down
(5)
Close temperature when sun goes down (5)
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The definition is at the end of the clue.
How the clue works
"When sun goes down" is your definition, pointing at night-time. Then for the wordplay, take NIGH (meaning "close") and stick T (the abbreviation for temperature) on the end, giving you the answer.