Nurse is working at break of day (7)
I believe the answer is:
sunrise
'break of day' is the definition.
(synonyms)
'nurse is working' is the wordplay.
'working' is an anagram indicator.
'nurse'+'is'='nurseis'
'nurseis' with letters rearranged gives 'SUNRISE'.
'at' acts as a link.
(Other definitions for sunrise that I've seen before include "Time of full daylight" , "What follows dawn" , "It gets the day off to a bright start" , "Dawning of the day" , "daily event" .)