Feasible, occasionally, to come up short (4)
I believe the answer is:
'to come up short' is the definition.
'feasible occasionally' is the wordplay.
'occasionally' indicates alternate letters.
The alternating letters of 'feasible' are 'FAIL'.
(Other definitions for fail that I've seen before include "Fall short" , "Run out; flop" , "Be unsuccessful, say in exam" , "Go wrong" , "Don't succeed" .)