After church, substantial breakfast dish (6)
I believe the answer is:
cereal
'breakfast dish' is the definition.
(I've seen this before)
'after church substantial' is the wordplay.
'after' means one lot of letters go next to another.
'church' becomes 'CE' (abbreviation for Church of England).
'substantial' becomes 'real' (synonyms).
'ce' put next to 'real' is 'CEREAL'.
(Other definitions for cereal that I've seen before include "Eg, wheat, rice" , "Maize, rye or the like" , "Eg, 5 Ac" , "Starchy grains used as food" , "Something for brekkie" .)