Open with part of Nessun dorma (4)
I believe the answer is:
undo
'open' is the definition.
(eg opening a bottle)
'part of nessun dorma' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'UNDO' can be found hidden inside 'nessun dorma'.
'with' acts as a link.
(Other definitions for undo that I've seen before include "Unfasten - destroy" , "Open, a knot say" , "Open button or ruin" , "Ruin prospects of; cancel" , "Cause the ruin of, or cancel" .)