If that doesn't work (4)
I believe the answer is:
else
'if that doesn't work' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for else that I've seen before include "Alternatively" , "Besides, otherwise" , "Adverb meaning if not, otherwise" , "As an alternative" , "Eels (anag)" .)