Iron part of the helmet altered (5)
I believe the answer is:
metal
'iron' is the definition.
(I've seen this before)
'part of the helmet altered' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'METAL' is hidden within 'helmet altered'.
(Other definitions for metal that I've seen before include "Aluminium or copper for instance" , "employed in making roads" , "Element such as iron, copper, zinc etc" , "Eg, titanium" , "Eg, Iron or gold" .)