A representative meeting before current measure (6)
I believe the answer is:
ampere
'current measure' is the definition.
(I've seen this before)
'a representative meeting before' is the wordplay.
'representative' becomes 'mp'.
'meeting' says to put letters next to each other.
'before' becomes 'ere' (usually literary).
'a'+'mp'+'ere'='AMPERE'
(Other definitions for ampere that I've seen before include "Basic SI unit of electric current" , "Solenoid inventor" , "Unit of currency" , "Unit measured by 17 across" , "Unit of electrical current" .)