Portion of vitamin I'm allowed is negligible (7)
I believe the answer is:
minimal
'negligible' is the definition.
(similar in meaning)
'portion of vitamin i'm allowed' is the wordplay.
'portion of' says the answer is hidden in the clue.
'MINIMAL' is hidden within 'vitamin im allowed'.
'is' is the link.
(Other definitions for minimal that I've seen before include "Least possible; negligible" , "barely worth considering" , "Token" , "Of a smallest amount" , "Very small, the least possible (7)" .)