If missing nightlife, lost temper (7)
I believe the answer is:
lighten
'temper' is the definition.
(thesaurus)
'if missing nightlife lost' is the wordplay.
'missing' suggests deleting specific letters.
'lost' indicates an anagram.
'nightlife' with 'if' taken out is 'nightle'.
'nightle' with letters rearranged gives 'LIGHTEN'.
(Other definitions for lighten that I've seen before include "lift" , "Relieve" , "Ease burden or reduce weight" , "make fairer" , "Make less serious" .)