Portion of filo I added, served up as garnish (5)
I believe the answer is:
aioli
'garnish' is the definition.
(I've seen this before)
'portion of filo i added served up' is the wordplay.
'portion of' indicates the answer is hidden within the clue.
'served up' says the letters should be written in reverse (in a down clue, letters go up).
'iloia' is hidden in the letters of 'filo i added'.
'iloia' back-to-front is 'AIOLI'.
'as' is the link.
(Other definitions for aioli that I've seen before include "mayonniase" , "Mayo, in part?" , "garlic sauce" , "Garlicky mayonnaise" , "Mayonnaise with garlic" .)