Periods when things look different (6)
I believe the answer is:
nights
'periods' is the definition.
(night is a kind of period)
'things look different' is the wordplay.
'look different' is an anagram indicator (I've seen 'looking different' mean this (I've seen this in other clues)).
'things' is an anagram of 'NIGHTS'.
'when' acts as a link.
(Other definitions for nights that I've seen before include "Periods of darkness" , "Periods after sunset" , "Times between days" , "Things (anag.)" , "Dark periods" .)