No male friend accepts resistance, as a rule (8)
I believe the answer is:
normally
'as a rule' is the definition.
(thesaurus)
'no male friend accepts resistance' is the wordplay.
'male' becomes 'm' (common abbreviation).
'friend' becomes 'ally' (I've seen this before).
'accepts' is an insertion indicator.
'resistance' becomes 'r' (abbreviation).
'no'+'m'+'ally'='nomally'
'nomally' placed around 'r' is 'NORMALLY'.
(Other definitions for normally that I've seen before include "Generally, usually" , "As a rule, usually" , "Ordinarily, usually" , "Usually, in the ordinary way" , "In the usual way" .)