Certainly that's in legal document (6)
I believe the answer is:
indeed
'certainly' is the definition.
(both can be used to answer in the affirmative)
'in legal document' is the wordplay.
'legal document' becomes 'deed' (deed is a kind of legal document).
'in'+'deed'='INDEED'
'that's' acts as a link.
(Other definitions for indeed that I've seen before include "Word expressing surprise, disbelief, etc" , "Unquestionably" , "Truthfully, really" , "Very much so" , "Without question" .)