Really it's in a legal document, though it could be denied (6)
I believe the answer is:
indeed
'really' is the definition.
(synonyms)
'could be denied' is the wordplay.
'could be' indicates anagramming the letters.
'denied' is an anagram of 'INDEED'.
'it's in a legal document though it' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
(Other definitions for indeed that I've seen before include "Really - what a surprise!" , "In truth, in fact" , "Absolutely" , "To be sure" , "In fact, really" .)