If not (6)
'if not' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for unless that I've seen before include "Conditional on" , "Except, save" , "Except on the condition that" , "Though" , "bar" .)
'if not' is the definition.
(synonyms)
This is all the clue.
(Other definitions for else that I've seen before include "In addition; different" , "Besides; different" , "Besides; instead" , "Otherwise; different from" , "In addition, besides" .)