Rescue silver stolen from barbarian (4)
I believe the answer is:
save
'rescue' is the definition.
(saving is a kind of rescuing)
'silver stolen from barbarian' is the wordplay.
'silver' becomes 'ag' ('Ag' can be a synonym of 'silver').
'stolen from' suggests deleting specific letters.
'barbarian' becomes 'savage' ('savage' can be a synonym of 'barbarian').
'savage' with 'ag' taken away is 'SAVE'.
(Other definitions for save that I've seen before include "Preserve; except" , "Be economical" , "Spend less" , "This may see ball tipped over it" , "Hang on to" .)