Cereal that's old at front of store (4)
I believe the answer is:
oats
'cereal' is the definition.
(type of cereal)
'old at front of store' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'front of' says to take the initial letters (the letter at the front of the word).
The initial letter of 'store' is 's'.
'o'+'at'+'s'='OATS'
'that's' is the link.
(Other definitions for oats that I've seen before include "Wild ones are sown in youthful excess" , "Cereal plants" , "Grain for porridge" , "Porridge ingredients" , "Grains or porridge" .)