Wrought steel, as in key (9)
I believe the answer is:
essential
'key' is the definition.
(I've seen this before)
'wrought steel as in' is the wordplay.
'wrought' indicates an anagram (wrought means beaten or shaped into form).
'steel'+'as'+'in'='steelasin'
'steelasin' with letters rearranged gives 'ESSENTIAL'.
(Other definitions for essential that I've seen before include "as oil might be" , "something necessary" , "Vitally important" , "So is certain oil-production" , "must-have item" .)