A light amount? (4)
I believe the answer is:
watt
'a light amount?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for watt that I've seen before include "Unit of power - Scottish inventor" , "Unit of electric power" , "Light bulb unit" , "Engineer, coined the term horsepower, d. 1819" , "James - - (Scottish inventor)" .)