Briefly fuse iron blade (5)
I believe the answer is:
'blade' is the definition.
(knife blade is a kind of blade)
'briefly fuse iron' is the wordplay.
'briefly' means to remove the last letter.
'fuse' becomes 'knit' (I've seen this before).
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'knit' with its last letter taken off is 'kni'.
(Other definitions for knife that I've seen before include "Sharp tool" , "Attack" , "Eating utensil" , "Cutting utensil" , "Cutting implement - stab" .)