Margherita-style portion is yummy (5)
I believe the answer is:
tasty
'yummy' is the definition.
(I've seen this in another clue)
'margherita-style portion' is the wordplay.
'portion' indicates the answer is hidden within the clue (I've seen 'portion of' mean this).
'TASTY' is hidden in the letters of 'margherita style'.
'is' acts as a link.
(Other definitions for tasty that I've seen before include "Pleasantly flavoursome" , "Palatable" , "Pleasant to savour" , "Attractive, appealing" , "Savoury" .)