Promise to keep omert? ultimately to Mafia's Don (5)
I believe the answer is:
swear
'promise' is the definition.
(thesaurus)
'ultimately to mafia's don' is the wordplay.
'ultimately' indicates one should take the final letters.
'don' becomes 'wear' ('wear' can be a synonym of 'don').
The final letter of 'mafias' is 's'.
's'+'wear'='SWEAR'
'to keep omert?' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for swear that I've seen before include "Assert" , "Solemnly promise" , "Asseverate" , "Eff and blind" , "not to mince one's words" .)