Duchess eats half of rare meat, which is a blow (6,4)
I believe the answer is:
karate chop
'a blow' is the definition.
(a karate chop is a blow or hit)
'duchess eats half of rare meat' is the wordplay.
'duchess' becomes 'kate' (Catherine, Duchess of Cambridge).
'eats' indicates putting letters inside.
'half of' indicates taking half.
'meat' becomes 'chop' (chop is a kind of meat).
'rare' cut in half is 'ra'.
'kate' enclosing 'ra' is 'karate'.
'karate'+'chop'='KARATE CHOP'
'which is' is the link.
(Other definitions for karate chop that I've seen before include "Hit with the side of the hand" , "strike from Japan" .)