Can of sauce (4)
I believe the answer is:
mayo
'sauce' is the definition.
'can of' is the wordplay.
'can' becomes 'may' (synonyms).
'of' becomes 'o''.
'may'+'o'='MAYO'
(Other definitions for mayo that I've seen before include "County of north-west Ireland" , "Cold, creamy sauce (abbr)" , "Irish county; dressing" , "Saucy thing" , "Salad dressing (informal)" .)