Crown court imprisons leaders of organised racket only (7)
I believe the answer is:
coronet
'crown' is the definition.
(coronet is a kind of crown)
'court imprisons leaders of organised racket only' is the wordplay.
'court' becomes 'ct'.
'imprisons' means one lot of letters goes inside another.
'leaders of' indicates taking the first letters.
'only' becomes 'one' ('one' can be a synonym of 'only'**).
The first letters of 'organised racket' is 'or'.
'or'+'one'='orone'
'ct' placed around 'orone' is 'CORONET'.
(Other definitions for coronet that I've seen before include "mark of noble rank?" , "jewelled headdress" , "sign of rank" , "badge of rank" , "in the band" .)