Beer down under is thin and metallic (5)
I believe the answer is:
tinny
I believe this clue is a double definition.
'beer down under' is the first definition.
(Australian slang for a can of beer)
'thin and metallic' is the second definition.
(I've seen this before)
'is' is the link.
(Other definitions for tinny that I've seen before include "With thin metallic sound" , "Metallic-sounding" , "Sounding thin and metallic" , "Aussie beer" , "Made of poor metal" .)