No marines join forces, by and large (8)
I believe the answer is:
normally
'by and large' is the definition.
(I've seen this before)
'no marines join forces' is the wordplay.
'marines' becomes 'rm' (abbreviation for 'Royal Marines').
'join forces' becomes 'ally' (I've seen this before).
'no'+'rm'+'ally'='NORMALLY'
(Other definitions for normally that I've seen before include "Mr Al Lyon? (anag)" , "In the ordinary or typical way" , "In the usual way" , "Usually, in the ordinary way" , "As a general rule" .)