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