For the most part, rule supports partner (8)
I believe the answer is:
normally
'for the most part' is the definition.
(thesaurus)
'rule supports partner' is the wordplay.
'rule supports' becomes 'norm' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'partner' becomes 'ally' (I've seen this in other clues).
'norm'+'ally'='NORMALLY'
Can you help me to learn more?
(Other definitions for normally that I've seen before include "Mr Al Lyon? (anag)" , "Ordinarily, usually" , "Generally, usually" , "As a general rule" , "In the usual or average situation" .)