Current measure a national leader set up (3)
I believe the answer is:
amp
'current measure' is the definition.
(short for ampere, unit of electrical current)
'a national leader set up' is the wordplay.
'national leader' becomes 'pm' (PM is an example. I am not sure about the 'national' bit.).
'set up' is a reversal indicator (in a down clue, letters go up).
'pm' reversed gives 'mp'.
'a'+'mp'='AMP'
Can you help me to learn more?
(Other definitions for amp that I've seen before include "Map of electric current" , "Unit of electricity, in short" , "Current measure" , "Certain amount of power" , "Device to up the sound of an electric guitar (abbr)" .)