Here’s politician before supporting a current measure (6)
I believe the answer is:
ampere
'current measure' is the definition.
(measure of electrical current)
'here's politician before supporting a' is the wordplay.
'here's politician' becomes 'mp' (I can't explain this - if you can you should give a lot more credence to this answer).
'before' becomes 'ere' (poetic term for 'before').
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'mp'+'ere'='mpere'
'mpere' after 'a' is 'AMPERE'.
(Other definitions for ampere that I've seen before include "Unit of electrical current" , "Unit of currency" , "Current measure" , "Current analyst" , "Basic SI unit of electric current" .)