Craze ultimately sure to decline (4)
I believe the answer is:
fade
'decline' is the definition.
(thesaurus)
'craze ultimately sure' is the wordplay.
'craze' becomes 'fad' ('fad' can be a synonym of 'craze').
'ultimately' indicates one should take the final letters.
The last letter of 'sure' is 'e'.
'fad'+'e'='FADE'
'to' acts as a link.
(Other definitions for fade that I've seen before include "Lose brightness" , "Deaf (anag.)" , "Gradually grow faint" , "Lose strength" , "Dwindle" .)