Potassium ruined fine blade (5)
I believe the answer is:
knife
'blade' is the definition.
(I know that knife is a type of blade)
'potassium ruined fine' is the wordplay.
'potassium' becomes 'K' (K is the chemical symbol for potassium).
'ruined' is an anagram indicator.
'fine' with letters rearranged gives 'nife'.
'k'+'nife'='KNIFE'
(Other definitions for knife that I've seen before include "Edge tool" , "Cutting utensil" , "Silent killer" , "Cut" , "Table implement" .)