A politician before current unit (6)
I believe the answer is:
ampere
'current unit' is the definition.
(ampere is a kind of current unit)
'a politician before' is the wordplay.
'politician' becomes 'MP' (Member of Parliament).
'before' becomes 'ere' (poetic term).
'a'+'mp'+'ere'='AMPERE'
(Other definitions for ampere that I've seen before include "Unit measured by 17 across" , "Current measure" , "scientist" , "Unit of currency" , "Solenoid inventor" .)