Raiders may be off with letter opener (4,3)
I believe the answer is:
dear sir
'letter opener' is the definition.
(I've seen this in another clue)
'raiders may be off' is the wordplay.
'raiders' anagrams to 'DEAR SIR'.
Though, I'm not sure how this is indicated.
'with' is the link.
Can you help me to learn more?
(Other definitions for dear sir that I've seen before include "Conventional letter opening to man" , "Opening words of formal letter" , "Form of address" , "Conventional opening address to male recipient of letter" , "Traditional intro" .)