Partisan could be one with ambition (8)
I believe the answer is:
aspirant
'one with ambition' is the definition.
(I've seen this before)
'partisan could be' is the wordplay.
'could be' indicates anagramming the letters.
'partisan' is an anagram of 'ASPIRANT'.
(Other definitions for aspirant that I've seen before include "Hopeful, wannabe" , "Partisan (anag)" , "Hopeful candidate" , "Hopeful one" , "Awannabee found in past rain" .)