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 unsure of the 'of armour' bit.
'vanadium is used before gold' is the wordplay.
'vanadium' becomes 'V' (chemical symbol).
'used before' is a charade indicator (letters next to each other).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'v'+'is'+'or'='VISOR'
'for' is the 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" .)