Dessert, when service should be given, we hear (6)
I believe the answer is:
sundae
'dessert' is the definition.
(sundae is a kind of dessert)
'when service should be given we hear' is the wordplay.
'when service should be given' becomes 'Sunday' (as in a church service).
'we hear' indicates a 'sounds like' (homophone) clue.
'sunday' sounds like 'SUNDAE'.
(Other definitions for sundae that I've seen before include "Ice cream served with fruit and syrup" , "Ice cream with toppings" , "Type of ice-cream" , "Confection for the weekend" , "A lush ice-cream - sounds Sabbatical" .)