Drug used in extremely severe case (8)
I believe the answer is:
sedative
'drug' is the definition.
(sedative is a kind of drug)
'extremely severe case' is the wordplay.
'extremely' means to remove the middle letters.
'case' becomes 'dative' (dative is a kind of case**).
'severe' with its middle removed is 'se'.
'se'+'dative'='SEDATIVE'
'used in' is the link.
(Other definitions for sedative that I've seen before include "Number" , "Dates I've changed for a tranquiliser" , "Tranquillising drug" , "Deviates from calming medicine" , "Drug to make one sleep" .)