Certainly fashionable to have legal document (6)
I believe the answer is:
indeed
'certainly' is the definition.
(thesaurus)
'fashionable to have legal document' is the wordplay.
'fashionable' becomes 'in' ('in' can be similar in meaning to 'fashionable').
'to have' says to put letters next to each other.
'legal document' becomes 'deed' (deed is a kind of legal document).
'in'+'deed'='INDEED'
(Other definitions for indeed that I've seen before include "Really - truly" , "beyond doubt" , "Expression of interest, incredulity or contempt" , "''A friend in need is a friend . . . . . .''" , "Very much so" .)