Can, perhaps, with old sauce (4)
I believe the answer is:
mayo
'sauce' is the definition.
(I know that mayo is a type of sauce)
'can perhaps with old' is the wordplay.
'can perhaps' becomes 'may' (synonyms. I am not sure about the 'perhaps' bit.).
'with' is a charade indicator (letters next to each other).
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'may'+'o'='MAYO'
Can you help me to learn more?
(Other definitions for mayo that I've seen before include "radio presenter" , "County of north-west Ireland" , "Eg, Braeburn or Cox" , "Irish county - sauce (abbr)" , "Irish county -- salad cream" .)