Prepares to fight when invaded by marines (4)
I believe the answer is:
arms
'prepares to fight' is the definition.
(to arm is to equip with weapons)
'when invaded by marines' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'invaded by' indicates putting letters inside.
'marines' becomes 'RM' (abbreviation for 'Royal Marines').
'as' enclosing 'rm' is 'ARMS'.
(Other definitions for arms that I've seen before include "Arsenal's items?" , "you'll find in the gunroom" , "bombs and guns etc" , "What the Venus de Milo lacks" , "Appendages" .)