Starchy cereal principally served in the past (4)
I believe the answer is:
sago
'starchy cereal' is the definition.
(I've seen this before)
'principally served in the past' is the wordplay.
'principally' indicates taking the first letters.
'in the past' becomes 'ago' (three years ago means three years in the past).
The initial letter of 'served' is 's'.
's'+'ago'='SAGO'
(Other definitions for sago that I've seen before include "Starchy pudding cereal" , "Palm cereal" , "Starchy grain used in desserts" , "Type of milk pudding" , "Edible palm starch" .)