Vanadium is used before gold for part of armour (5)
I believe the answer is:
visor
'part of armour' is the definition.
'visor' can be an answer for 'part' (I have seen 'Part of helmet' mean 'visor' so perhaps 'part' could also mean 'visor'). I'm not certain of the 'of armour' bit.
'vanadium is used before gold' is the wordplay.
'vanadium' becomes 'V' (V is the chemical symbol for vanadium).
'used before' means one lot of letters go next to another.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'v'+'is'+'or'='VISOR'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for visor that I've seen before include "Front piece of a helmet" , "Transparent flap on a helmet" , "Eye protector" , "Face-armour" , "Face armour" .)