Lightbulb measure (4)
I believe the answer is:
watt
'lightbulb measure' 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 "Worker in steam" , "SI unit of power" , "Work rate" , "Unit of power -- Scottish engineer, d. 1819" , "Scottish engineer and inventor, d. 1819" .)