Discredit official before cup tie? Only occasionally (6)
I believe the answer is:
refute
'discredit' is the definition.
(to refute is to discredit or debunk)
'official before cup tie? only occasionally' is the wordplay.
'official' becomes 'ref' (match official in football, say).
'before' says to put letters next to each other.
'only occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'cup'+'tie'='cuptie'
The alternating letters of 'cuptie' are 'ute'.
'ref'+'ute'='REFUTE'
(Other definitions for refute that I've seen before include "Prove something to be wrong" , "Disprove" , "Contradict, prove that statement is wrong" , "Prove wrong; deny" , "Prove to be incorrect" .)