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' (abbreviation).
'front of' indicates taking the first letters (the letter at the front of the word).
The first letter of 'store' is 's'.
'o'+'at'+'s'='OATS'
'that's' acts as a 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" .)