Amount — portion (7)
I believe the answer is:
quantum
I believe this clue is a double definition.
'amount' is the first definition.
(I know that quantum is a type of amount)
'portion' is the second definition.
(thesaurus)
(Other definitions for quantum that I've seen before include "Type of mechanics, jump or leap" , "Specific amount - type of mechanics" , "Amount - measure" , "... of Solace, James Bond film" , "Smallest possible amount (eg of energy) - large and sudden" .)