As a rule, no soldiers band together (8)
I believe the answer is:
normally
'as a rule' is the definition.
(I've seen this before)
'no soldiers band together' is the wordplay.
'soldiers' becomes 'RM' (Royal Marines).
'band together' becomes 'ally' (I've seen this before).
'no'+'rm'+'ally'='NORMALLY'
(Other definitions for normally that I've seen before include "In the ordinary or typical way" , "Ordinarily, usually" , "Naturally" , "As a general rule" , "Generally, usually" .)