Two metals forming obstacle (4)
I believe the answer is:
snag
'obstacle' is the definition.
(I know that snag is a type of obstacle)
'two metals' is the wordplay.
'two metals' can mean two replacements for 'metal'.
'metal' becomes 'sn' (tin).
'metal' becomes 'ag' (silver).
'sn'+'ag'='SNAG'
'forming' acts as a link.
(Other definitions for snag that I've seen before include "Unforeseen obstacle" , "Temporary hold-up" , "Hidden obstacle or drawback" , "Glitch" , "Jag" .)