Dear Sir, perhaps a paper knife? (6,6)
I believe the answer is:
letter opener
'paper knife?' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for letter opener that I've seen before include "Machine for dealing with the post" , "Implement for tearing envelopes" , "which could be dear, eg" , "Desktop tool" , "Dear (usually)" .)