Finest metal for big military wheels (3,5)
I believe the answer is:
top brass
'big military wheels' is the definition.
'finest metal' is the wordplay.
'finest' becomes 'top' (I've seen this before).
'metal' becomes 'brass' (brass is a kind of metal**).
'top'+'brass'='TOP BRASS'
'for' acts as a link.
(Other definitions for top brass that I've seen before include "Authorities" , "senior command" , "The people in charge (of the Army?)" , "management" , "People in authority (informal)" .)