Other (4)
(Other definitions for alternative that I've seen before include "One of a number of possibilities" , "(Of two things) mutually exclusive" , "One possible option" , "Substitute" , "One of two or more available possibilities" .)
'other' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for else that I've seen before include "Alternatively" , "Adverb meaning if not, otherwise" , "In addition; different" , "In addition, besides" , "Other than (what is under consideration)" .)