Without bias, I belong with one partner (11)
I believe the answer is:
impartially
'without bias i' is the definition.
I can't tell whether this definition defines the answer.
'belong with one partner' is the wordplay.
'belong with' becomes 'impart' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'one' becomes 'I' (Roman numeral).
'partner' becomes 'ally' (I've seen this in another clue).
'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" .)