Capone's neckwear expert (8)
I believe the answer is:
scarface
'capone's' is the definition.
(Al Capone was nicknamed Scarface)
'neckwear expert' is the wordplay.
'neckwear' becomes 'scarf' (I've seen this before).
'expert' becomes 'ace' (ace can informally mean very good).
'scarf'+'ace'='SCARFACE'
(Other definitions for scarface that I've seen before include "Al Capone, or Tony Montana - 'Say hello to my little friend'." , "American gangster" , "So-called gangster" , "US gangster" , "Film starring Al Pacino" .)