Serial heard at breakfast, perhaps? (6)
I believe the answer is:
cereal
'breakfast perhaps' is the definition.
(I know that breakfast is a type of grist)
'serial heard' is the wordplay.
'heard' shows a homophone (sound like).
'serial' sounds like 'CEREAL'.
'at' acts as a link.
(Other definitions for cereal that I've seen before include "Something for brekkie" , "Grain crop" , "Grass producing an edible grain" , "Breakfast food made from grain" , "Contents of breakfast bowl" .)