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' means one lot of letters go next to another.
'before' becomes 'ere' (poetic term for 'before').
'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" .)