Month-old sauce (4)
I believe the answer is:
mayo
'sauce' is the definition.
(I know that mayo is a type of sauce)
'month-old' is the wordplay.
'month' becomes 'may' (May is an example).
'old' becomes 'o' (abbreviation).
'may'+'o'='MAYO'
(Other definitions for mayo that I've seen before include "Irish county (county town, Castlebar)" , "You'll find it in the Emerald Isle" , "radio presenter" , "part of Ireland" , "Saucy thing" .)