Fighting force always includes Royal Marines (4)
I believe the answer is:
army
'fighting force' is the definition.
(I've seen this before)
'always includes royal marines' is the wordplay.
'always' becomes 'ay'.
'includes' is an insertion indicator.
'royal marines' becomes 'rm'.
'ay' placed around 'rm' is 'ARMY'.
(Other definitions for army that I've seen before include "See 7" , "Vast number" , "fighters" , "Large number (of ants?)" , "A defence force" .)