Vehicle engine cover half made of such steel? (6)
I believe the answer is:
carbon
'such steel?' is the definition.
(carbon steel is a type of steel)
'vehicle engine cover half' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'engine cover' becomes 'bonnet' (I've seen this before).
'half' indicates taking half.
'bonnet' cut in half is 'bon'.
'car'+'bon'='CARBON'
'made of' acts as a link.
(Other definitions for carbon that I've seen before include "sort of copy" , "C (with a footprint)" , "Chemical element occurring in pure form in diamonds and graphite" , "Chemical element occurring in pure form in diamond" , "Element of which diamonds are formed" .)