Crossword Genius

Certainly that's in legal document (6)

Ross

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" .)

I've seen this clue in The Times and The Sun.
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