Unequivocal, in writing with one hand (9)
I believe the answer is:
downright
'unequivocal' is the definition.
(thesaurus)
'in writing with one hand' is the wordplay.
'in writing' becomes 'down' (as in written down).
'with' says to put letters next to each other.
'one hand' becomes 'right' (right is a kind of hand).
'down'+'right'='DOWNRIGHT'
(Other definitions for downright that I've seen before include "Utter, complete" , "Frank" , "Unequivocally" , "Blatant" , "Utter, arrant" .)