Bishop first to eat egg on the counter, most substantial (7)
I believe the answer is:
biggest
'most substantial' is the definition.
(big can mean substantial)
'bishop first to eat egg on the counter' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'first to' becomes 'ist' (I've seen this in other clues).
'eat' indicates putting letters inside.
'on the counter' says the letters should be written backwards (counter can mean in the opposite direction).
'egg' in reverse letter order is 'gge'.
'ist' placed around 'gge' is 'iggest'.
'b'+'iggest'='BIGGEST'
(Other definitions for biggest that I've seen before include "Superlatively large" , "Largest" , "Greatest in size" , "Extremely great" .)