Tape of extraordinary test case (8)
I believe the answer is:
cassette
'tape' is the definition.
(cassette tape is a kind of tape)
'extraordinary test case' is the wordplay.
'extraordinary' indicates anagramming the letters.
'test'+'case'='testcase'
'testcase' anagrammed gives 'CASSETTE'.
'of' is the link.
(Other definitions for cassette that I've seen before include "Tape format" , "Container to go into tape deck" , "Case for reel of magnetic tape" , "Music tape" , "Vintage recording" .)