King and knight with one iron blade (5)
I believe the answer is:
knife
'blade' is the definition.
(knife blade is a kind of blade)
'king and knight with one iron' is the wordplay.
'king' becomes 'K' (abbreviation for king in chess).
'and' means one lot of letters go next to another.
'knight' becomes 'n' (chess abbreviation).
'with one' becomes 'i' (Roman numeral. I am not sure about the 'with' bit.).
'iron' becomes 'fe' (Fe is the chemical symbol for iron).
'k'+'n'+'i'+'fe'='KNIFE'
Can you help me to learn more?
(Other definitions for knife that I've seen before include "following Stanley perhaps" , "Table implement" , "Use this to carve" , "Attack" , "Cutting tool" .)