Save Royal Marines crossing European river (6)
I believe the answer is:
redeem
'save' is the definition.
(I know this)
'royal marines crossing european river' is the wordplay.
'royal marines' becomes 'rm'.
'crossing' means one lot of letters goes inside another.
'european' becomes 'E' (abbreviation e.g. EU).
'river' becomes 'dee' (River Dee).
'e'+'dee'='edee'
'rm' going around 'edee' is 'REDEEM'.
(Other definitions for redeem that I've seen before include "rescue" , "Avoid pawn loss" , "Exchange (coupon) for cash or goods" , "Get back" , "Recover from the pawnshop" .)