Metal seat at Metro is oddly missing (6)
I believe the answer is:
pewter
'metal' is the definition.
(pewter is a kind of metal)
'seat at metro is oddly missing' is the wordplay.
'seat' becomes 'pew' (pew is a kind of seat**).
'is oddly missing' means one should take alternating letters.
'at'+'metro'='atmetro'
The alternate letters of 'atmetro' are 'ter'.
'pew'+'ter'='PEWTER'
(Other definitions for pewter that I've seen before include "Alloy of tin with copper and antimony" , "Grey alloy of tin and lead, makes mugs etc." , "Grey alloy of tin and lead, often in mug" , "Alloy used for utensils" , "Tin alloy used to make plates and mugs" .)