The Saab doesn't need a wash, it's a day of rest! (7)
I believe the answer is:
sabbath
'a day of rest' is the definition.
(Sabbath is an example)
'the saab doesn't need a wash' is the wordplay.
'doesn't need' indicates named letters should be taken away.
'wash' becomes 'bath' (bath is a kind of wash).
'saab' with 'a' taken out is 'sab'.
'sab'+'bath'='SABBATH'
'it's' is the link.
(Other definitions for sabbath that I've seen before include "Religious day of rest for Jews and Christians" , "Traditional holy day" , "Hebrew word for day of rest" , "Period of rest" , "Jewish rest-day" .)