Tape group, finally -- just in case (8)
I believe the answer is:
cassette
'tape' is the definition.
(cassette tape is a kind of tape)
'group finally just in case' is the wordplay.
'group' becomes 'set' (both can mean a collection).
'finally' suggests the final letters.
'in' means one lot of letters goes inside another.
The final letter of 'just' is 't'.
'set'+'t'='sett'
'sett' placed within 'case' is 'CASSETTE'.
(Other definitions for cassette that I've seen before include "Test case about tape container" , "Music tape" , "Small sealed case of magnetic tape" , "Tape format" , "Source of music" .)