Conceal some grave illness (4)
I believe the answer is:
veil
'conceal' is the definition.
(I know that veil is a type of conceal)
'some grave illness' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'VEIL' is hidden within 'grave illness'.
(Other definitions for veil that I've seen before include "Lacy facial covering" , "Concealing material" , "Face concealer" , "Face-covering for brides and other women" , "Bridal headdress" .)