Sauce, month old (4)
I believe the answer is:
mayo
'sauce' is the definition.
'month old' is the wordplay.
'month' becomes 'may' (May is an example**).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'may'+'o'='MAYO'
(Other definitions for mayo that I've seen before include "part of Ireland" , "Saucy thing" , "Irish county; dressing" , "Cold, creamy sauce (abbr)" , "Eg, Braeburn or Cox" .)