Old saying of a cutting nature (3)
I believe the answer is:
saw
'old saying' is the definition.
(I've seen this in another clue)
'a cutting nature' is the wordplay.
'cutting' indicates putting letters inside (some letters cut their way into a word).
'nature' becomes 'sw' (I can't explain this - if you can you should believe this answer much more).
'a' put inside 'sw' is 'SAW'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for saw that I've seen before include "See 23" , "Old saying with teeth" , "Perceived visually" , "Spotted" , "Implement for cutting wood" .)