Indispensable, as in steel construction (9)
I believe the answer is:
essential
'indispensable' is the definition.
('essential' can be a synonym of 'indispensable')
'as in steel construction' is the wordplay.
'construction' indicates anagramming the letters (answer constructed from certain letters).
'as'+'in'+'steel'='asinsteel'
'asinsteel' is an anagram of 'ESSENTIAL'.
(Other definitions for essential that I've seen before include "must-have item" , "absolutely neccessary?" , "Key" , "Vitally important" , "Indispensible" .)