Lord's day of rest (7)
I believe the answer is:
sabbath
'lord's day of rest' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sabbath that I've seen before include "Day of rest and worship" , "Period of rest" , "Religious day of rest for Jews and Christians" , "Jewish rest-day" , "Traditional holy day" .)