Sauce? Feel bad after ingesting old one (5)
I believe the answer is:
aioli
'sauce?' is the definition.
(aioli is a kind of sauce)
'feel bad after ingesting old one' is the wordplay.
'feel bad' becomes 'ail' (I've seen this in another clue).
'after ingesting' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'one' becomes 'i' (Roman numeral).
'ail' placed around 'o' is 'aiol'.
'aiol'+'i'='AIOLI'
(Other definitions for aioli that I've seen before include "Dressing" , "Mayo, in part?" , "mayonniase" , "11 [SAUCE]" , "Pungent sauce" .)