Firm beginning to build key metallic element (6)
I believe the answer is:
cobalt
'metallic element' is the definition.
(I know that cobalt is a type of metallic element)
'firm beginning to build key' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'beginning to' says to take the initial letters.
'key' becomes 'alt' (the Alt keyboard button).
The initial letter of 'build' is 'b'.
'co'+'b'+'alt'='COBALT'
(Other definitions for cobalt that I've seen before include "Element 27; a deep blue" , "Metal element" , "Bright blue pigment" , "Ferromagnetic metal - shade of blue" , "Sort of blue" .)