Normand — one in supporting role as a rule (8)
I believe the answer is:
normally
'as a rule' is the definition.
(I've seen this before)
'normand one in supporting role' is the wordplay.
'normand' becomes 'norm' (I can't explain this - if you can you should give a lot more credence to this answer).
'one in supporting role' becomes 'ally' (an ally offers support).
'norm'+'ally'='NORMALLY'
(Other definitions for normally that I've seen before include "As a rule, usually" , "in standard fashion" , "In the ordinary or typical way" , "Generally, usually" , "By and large" .)