Stump set by umpire, occasionally (6)
I believe the answer is:
stymie
'stump' is the definition.
(I've seen this in another clue)
'set by umpire occasionally' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'set'+'by'+'umpire'='setbyumpire'
The alternate letters of 'setbyumpire' are 'STYMIE'.
(Other definitions for stymie that I've seen before include "Block or thwart, perhaps in golf" , "'Hinder, prevent progress (6)'" , "Block; hinder" , "balk" , "Frustrate" .)