Word in an ultimatum (6)
(Other definitions for unless that I've seen before include "Except, save" , "Conjunction meaning except when" , "Otherwise than if" , "bar" , "Though" .)
'word in an ultimatum' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for else that I've seen before include "Alternatively" , "Eels (anag)" , "Besides; instead" , "Besides; different" , "In addition; different" .)