Essentially, newcomer tackles code of silence (6)
I believe the answer is:
omerta
'code of silence' is the definition.
(code of silence in the Mafia)
'essentially newcomer tackles' is the wordplay.
'essentially' indicates the answer is hidden within the clue (essence can mean the inside of something).
'OMERTA' is hidden in the letters of 'newcomer tackles'.
(Other definitions for omerta that I've seen before include "Mafia pledge of silence" , "Mafia code of silence; at Rome (anag.)" , "conspiracy of silence" , "vow of silence" , "Mafia code of silence, never inform" .)