Mephisto

[Totterdown]

2D - Head honcho in Cosa Nostra with something to grip neck tightly (4) CAP? I guess the answer is cape, but can someone explain how from the clue? Thanks.

[dr b]

capo; both a Mafia boss and "a movable bridge secured over the fingerboard and strings of a lute or guitar, to alter the pitch of all the strings together"

[kettledrum]

capo ... head of a branch of the Mafia (Cosa Nostra), and a 'thing' that grips the neck (and strings) of a lute or guitar to alter the pitch of ALL the strings together.

[Totterdown]

Gosh, I wouldn't have got that on my own. Thank you very much dr b.

[Beatrice35]

Capo- a clamp fasten across strings of a fretted musical instrument. Neck of the guitar.
Italian capo Tasto- head stop
Honcho- Leader, to be in charge of ( Head)

Hope the above helps, Bea, night night :-)

[Totterdown]

Also thanks to kettledrum and Beatrice 35. Your answers crossed with mine.