Current unit a member gets before (6)
I believe the answer is:
ampere
'current unit' is the definition.
(ampere is a kind of current unit)
'a member gets before' is the wordplay.
'member' becomes 'MP' (Member of Parliament).
'gets' is a charade indicator (letters next to each other).
'before' becomes 'ere' (poetic term).
'a'+'mp'+'ere'='AMPERE'
(Other definitions for ampere that I've seen before include "Unit of electrical current" , "Current analyst" , "Unit measured by 17 across" , "Basic SI unit of electric current" , "Unit of currency" .)