Successfully defend finale to ITV show (9)
I believe the answer is:
vindicate
'successfully defend' is the definition.
(I've seen this before)
'finale to itv show' is the wordplay.
'finale' says to take the final letters.
'show' becomes 'indicate' (synonyms).
The final letter of 'itv' is 'v'.
'v'+'indicate'='VINDICATE'
(Other definitions for vindicate that I've seen before include "Deflect criticism" , "support" , "Divine cat (anag.)" , "Avenge or exonerate" , "Justify by evidence or results" .)