Arcade nearly collapsing — its days may be numbered (8,4)
I believe the answer is:
calendar year
'its days may be numbered' is the definition.
I can't tell whether this definition defines the answer.
'arcade nearly collapsing' is the wordplay.
'collapsing' is an anagram indicator.
'arcade'+'nearly'='arcadenearly'
'arcadenearly' is an anagram of 'CALENDAR YEAR'.
Can you help me to learn more?
(Other definitions for calendar year that I've seen before include "Twelve months" , "period of months?" , "January 1 to December 31 inclusive" .)