Assassinates duchess with steel-bladed boot (3,5)
I believe the answer is:
ice skate
'boot' is the definition.
(I've seen this before)
'assassinates duchess with' is the wordplay.
'assassinates' becomes 'ices' (slang term).
'duchess with' becomes 'kate' (I can't justify this - if you can you should believe this answer much more).
'ices'+'kate'='ICE-SKATE'
'steel-bladed' is the link.
This may not be correct. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for ice skate that I've seen before include "Glide over frozen water" , "it makes progress in an entertainment centre" , "Use blade to go" , "Attachment to a winter sport's boot" , "Shoe with runner" .)