Tenor joins singing group during quiet day off (7)
I believe the answer is:
sabbath
'day off' is the definition.
(day off work)
'tenor joins singing group during quiet' is the wordplay.
'tenor' becomes 't'.
'joins' says to put letters next to each other.
'singing group' becomes 'ABBA' (ABBA are a band).
'during' indicates putting letters inside.
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
't' put after 'abba' is 'abbat'.
'abbat' inserted within 'sh' is 'SABBATH'.
(Other definitions for sabbath that I've seen before include "Jewish rest-day" , "Day of rest and worship" , "Period of rest" , "Sacred day of the week" , "Traditional holy day" .)