Old video format prescribed in class (8)
I believe the answer is:
cassette
'old video format' is the definition.
(video cassette)
'prescribed in class' is the wordplay.
'prescribed' becomes 'set' (set can mean pre-determined or prescribed).
'in' means one lot of letters goes inside another.
'class' becomes 'caste' (type of social class).
'set' inserted within 'caste' is 'CASSETTE'.
(Other definitions for cassette that I've seen before include "Tape unit" , "Test case about tape container" , "Recording medium" , "Spool of tape" , "Recorder would accommodate this" .)