Beer down under is thin and metallic (5)
I believe the answer is:
Here is my best explanation:
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 "Shoddy", "Of poor audio quality", "Made of poor metal", "With thin metallic sound", "appear so badly constructed?", "Aussie beer", "Lacking resonance", "sounding metallic", "Metallic in sound" and "Cheap".)