Disheartened boy admits in retrospect virtually brilliant crime (6)
I believe the answer is:
bigamy
'crime' is the definition.
(bigamy is a kind of crime)
'disheartened boy admits in retrospect virtually brilliant' is the wordplay.
'disheartened' says to hollow out the word (remove centre letters).
'admits' means one lot of letters goes inside another.
'in retrospect' is a reversal indicator.
'virtually brilliant' becomes 'magi' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'boy' with its centre removed is 'by'.
'magi' back-to-front is 'igam'.
'by' placed around 'igam' is 'BIGAMY'.
Can you help me to learn more?
(Other definitions for bigamy that I've seen before include "Crime of double marriage" , "becoming a member of two unions?" , "Having more than one spouse, illegally" , "overmatiness" , "Crime of marrying when already legally wed" .)