Physical amount of energy left over in New York (7)
I believe the answer is:
entropy
'physical amount' is the definition.
(physics term)
'energy left over in new york' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mco).
'left' becomes 'port' (left side of a ship - opposite of starboard).
'over' says the letters should be written backwards.
'in' is an insertion indicator.
'new york' becomes 'ny'.
'port' back-to-front is 'trop'.
'trop' going into 'ny' is 'ntropy'.
'e'+'ntropy'='ENTROPY'
'of' is the link.
(Other definitions for entropy that I've seen before include "Random quality" , "Physical quantity linked to disorder" , "Physical state" , "Term in physics" , "Degree of disorder (physics)" .)