Without bias, I belong with one partner (11)
I believe the answer is:
impartially
'without bias i' is the definition.
I can't judge whether this definition defines the answer.
'belong with one partner' is the wordplay.
'belong with' becomes 'impart' (I can't justify this - if you can you should believe this answer much more).
'one' becomes 'i' (Roman numeral).
'partner' becomes 'ally' (an ally is an associate or supporter).
'impart'+'i'+'ally'='IMPARTIALLY'
Can you help me to learn more?
(Other definitions for impartially that I've seen before include "In an even-handed manner" , "So as to treat both sides equally" .)