A legislator's current measure (3)
I believe the answer is:
amp
'current measure' is the definition.
(unit of electrical current)
'a legislator's' is the wordplay.
'legislator' becomes 'MP' (an MP legislates in parliament).
'a'+'mp'='AMP'
(Other definitions for amp that I've seen before include "Device making sound louder" , "Device to up the sound of an electric guitar (abbr)" , "sound is increased by this" , "Physics unit" , "Certain amount of power" .)