Most prominent metal in building, ultimately (7)
I believe the answer is:
leading
'most prominent' is the definition.
'leading' can be an answer for 'prominent' (I've seen this before). I'm unsure of the 'most' bit.
'metal in building ultimately' is the wordplay.
'metal' becomes 'lead' (lead is a kind of metal).
'building ultimately' becomes 'g' (I can't explain this - if you can you should believe this answer much more).
'lead'+'in'+'g'='LEADING'
Can you help me to learn more?
(Other definitions for leading that I've seen before include "In the vanguard" , "Principal" , "Female top of the bill" , "Prominent" , "'Showing the way, out in front (7)'" .)