It's a different thing when it falls (5)
I believe the answer is:
night
'it falls' is the definition.
The definition suggests a singular noun which matches the answer.
'it's a different thing' is the wordplay.
'it's a different' is an anagram indicator (letters placed in a different order).
'thing' is an anagram of 'NIGHT'.
'when' is the link.
Can you help me to learn more?
(Other definitions for night that I've seen before include "Day's partner" , "Time between days" , "Dark hours" , "late" , "follow-up to 16 [TWILIGHT]" .)