Test hard material lacking carbon (4)
I believe the answer is:
oral
'test' is the definition.
(oral is a type of examination)
'hard material lacking carbon' is the wordplay.
'hard material' becomes 'coral' (I've seen this before).
'lacking' indicates named letters should be taken away.
'carbon' becomes 'C' (C is the chemical symbol for carbon).
'coral' with 'c' taken away is 'ORAL'.
(Other definitions for oral that I've seen before include "Of the voice" , "See 18" , "Like some tradition" , "By word of mouth (4)" , "Related to the mouth" .)