Surely that could be denied (6)
I believe the answer is:
indeed
'surely' is the definition.
(I've seen this before)
'could be denied' is the wordplay.
'could be' indicates anagramming the letters.
'denied' is an anagram of 'INDEED'.
'that' acts as a link.
(Other definitions for indeed that I've seen before include "Surely" , "Absolutely" , "Really, actually" , "Really - truly" , "Word expressing surprise, disbelief, etc" .)