They're eating their main meals without heart (6)
I believe the answer is:
diners
'they're eating their main' is the definition.
The definition suggests an adverb but the answer is not.
'meals without heart' is the wordplay.
'meals' becomes 'dinners' (dinner is a kind of meal).
'without heart' means to remove the middle letters.
'dinners' with its centre removed is 'DINERS'.
Can you help me to learn more?
(Other definitions for diners that I've seen before include "Eaters of a meal" , "Restaurants" .)