Tin alloy (6)
I believe the answer is:
'tin alloy' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for pewter that I've seen before include "An alloy of tin with a small amount of other metal" , "Grey alloy of tin and lead, makes mugs etc." , "Alloy of tin and lead, often in beer mug" , "Tin alloy used to make plates and mugs" , "prize-money" .)