Besides, otherwise (4)
I believe the answer is:
else
This clue is a double definition.
'besides' is the first definition.
(I've seen this in another clue)
'otherwise' is the second definition.
(synonyms)
(Other definitions for else that I've seen before include "In addition; different" , "Besides, otherwise" , "As an alternative" , "In addition, besides" , "Alternatively" .)