Drug used in extremely strange case (8)
I believe the answer is:
sedative
'drug' is the definition.
(sedative is a kind of drug)
'extremely strange case' is the wordplay.
'extremely' means to remove the middle letters.
'case' becomes 'dative' (dative is a kind of case**).
'strange' with its middle taken out is 'se'.
'se'+'dative'='SEDATIVE'
'used in' is the link.
(Other definitions for sedative that I've seen before include "Dates I've changed for a tranquiliser" , "Sleeping drug" , "Sleep-inducing drug" , "Drug to make one sleep" , "Tranquillising drug" .)