Cultivated TV voice describing start of abnormal case … (8)
I believe the answer is:
'case' is the definition.
(vocative is a kind of case)
'cultivated tv voice describing start of abnormal' is the wordplay.
'cultivated' indicates an anagram.
'describing' indicates putting letters inside (describe can mean to form the outline of).
'start of' says to take the initial letters.
The first letter of 'abnormal' is 'a'.
'tvvoice' with letters rearranged gives 'voctive'.
'voctive' placed around 'a' is 'VOCATIVE'.
(Another definition for vocative that I've seen is " Addressing case (gram.)".)