An amount (7)
I believe the answer is:
quantum
'an amount' is the definition.
('quantum' can be a synonym of 'amount')
This is the entire clue.
(Other definitions for quantum that I've seen before include "Smallest possible amount (eg of energy) - large and sudden" , "Amount - measure" , "one tiny jump for a physicist" , "Small discrete amount" , "... of Solace, James Bond film" .)