As a rule, no marines join forces (8)
I believe the answer is:
normally
'as a rule' is the definition.
(I've seen this in another clue)
'no marines join forces' is the wordplay.
'marines' becomes 'RM' (abbreviation for 'Royal Marines').
'join forces' becomes 'ally' (to ally with someone is to join forces with them).
'no'+'rm'+'ally'='NORMALLY'
(Other definitions for normally that I've seen before include "in standard fashion" , "Usually, in the ordinary way" , "In the usual manner" , "In the usual way" , "In the usual or average situation" .)