Royal Marines east of area with our protective clothing (6)
I believe the answer is:
armour
'protective clothing' is the definition.
(I've seen this before)
'royal marines east of area with our' is the wordplay.
'royal marines' becomes 'RM' (abbreviation).
'east of' is a charade indicator (letters next to each other) (some letters go to the right of others).
'area' becomes 'a' (maths abbreviation).
'with' means one lot of letters go next to another.
'rm' after 'a' is 'arm'.
'arm'+'our'='ARMOUR'
(Other definitions for armour that I've seen before include "Protective covering for knight of old" , "Body plate" , "defence" , "Protective covering for the body" , "Protective apparel" .)