A student fad mainly about a time in prison once (8)
I believe the answer is:
alcatraz
'prison once' is the definition.
(historical prison)
'a student fad mainly about a time' is the wordplay.
'student' becomes 'l' (as in L-plates for learner drivers).
'fad' becomes 'craze' (synonyms).
'mainly' means to remove the last letter (virtually or mainly the word, but not all of it).
'about' is an insertion indicator.
'a time' becomes 'at' (I can't explain this - if you can you should give a lot more credence to this answer).
'craze' with its last letter taken off is 'craz'.
'craz' enclosing 'at' is 'catraz'.
'a'+'l'+'catraz'='ALCATRAZ'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for alcatraz that I've seen before include "that caged bird mate?" , "Californian island" , "US island of a former maximum-security prison" , "Old Calif. offshore prison" , "Birdman's prison" .)