Calming extremes of sensitive case (8)
I believe the answer is:
sedative
'calming' is the definition.
(synonyms)
'extremes of sensitive case' is the wordplay.
'extremes of' suggests removing the centre (only the letters at the extremes of the word).
'sensitive' becomes 'sore' (as in a sore wound).
'case' becomes 'dative' (the dative case in grammar).
'sore' with its centre removed is 'se'.
'se'+'dative'='SEDATIVE'
(Other definitions for sedative that I've seen before include "Tranquilliser" , "quieter" , "Depressant" , "Dates I've changed for a tranquiliser" , "Deviates from calming medicine" .)