Excellent companion -- a politician and I will get on (8)
I believe the answer is:
champion
'excellent' is the definition.
(I've seen this before)
'companion a politician and i will get on' is the wordplay.
'companion' becomes 'cha' (I've seen this in other clues).
'a politician' becomes 'mp'.
'and' says to put letters next to each other.
'will get' says to put letters next to each other.
'cha'+'mp'+'i'+'on'='CHAMPION'
(Other definitions for champion that I've seen before include "1st place winner" , "Winner; defender" , "Defender of a cause" , "Campaigner or winner" , "Tournament winner" .)