Sources of any largely uncommon metal provide metallic salt (4)
I believe the answer is:
alum
'salt' is the definition.
(I know that alum is a type of double salt)
'sources of any largely uncommon metal' is the wordplay.
'sources of' indicates taking the first letters (the beginning of a word is its sauce).
The first letters of 'any largely uncommon metal' is 'ALUM'.
'provide metallic' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for alum that I've seen before include "White mineral salt used in medicine and in dyeing" , "Colourless compound used in dyeing and tanning" , "Name given to a specific chemical compound and a class of them" , "Aluminium/potassium sulphate" , "Potassium compound; IoW sandy bay" .)