Prefix with partisan (3)
I believe the answer is:
non
'prefix with partisan' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for non that I've seen before include "Prefix meaning not or none" , "Prefixed negation of a word" , "- - -stick" , "Negative French word" , "Prefix meaning negative" .)