Day of rest sounds sweet (6)
I believe the answer is:
sundae
'sweet' is the definition.
(sundae is a kind of sweet)
'day of rest sounds' is the wordplay.
'day of rest' becomes 'sunday' (Sunday is an example).
'sounds' indicates a 'sounds like' (homophone) clue.
'sunday' sounds like 'SUNDAE'.
(Other definitions for sundae that I've seen before include "Ice cream with toppings" , "Ice cream with syrup or crushed fruit" , "Rich ice cream dish" , "Ice cream dessert" , "'Ice cream topped with fruit, nuts and syrup (6)'" .)