Justified in changing their night (2,3,5)
I believe the answer is:
in the right
'justified' is the definition.
(I've seen this before)
'changing their night' is the wordplay.
'changing' is an anagram indicator.
'their'+'night'='theirnight'
'theirnight' anagrammed gives 'IN THE RIGHT'.
'in' is the link.
(Other definitions for in the right that I've seen before include "justified" , "Not wrong" , "Maintaining a justifiable position" , "Conservatives" , "Supported by law (or reason)" .)