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' is an anagram indicator.
'denied' with letters rearranged gives 'INDEED'.
'it's in a legal document though it' is the 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" .)