Dark times — things must be altered (6)
I believe the answer is:
nights
'dark times' is the definition.
(it's dark at night)
'things must be altered' is the wordplay.
'must be altered' indicates an anagram.
'things' anagrammed gives 'NIGHTS'.
(Other definitions for nights that I've seen before include "Dark periods" , "Periods of darkness" , "Periods after sunset" , "See 9" , "Things (anag.)" .)