Cunning one using one cunningly (9)
I believe the answer is:
ingenious
'cunning' is the definition.
(I know that cunning can be written as ingenious)
'one using one cunningly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'cunningly' indicates anagramming the letters (a cunning or tricky form of the letters).
'using'+'one'='usingone'
'usingone' with letters rearranged gives 'ngenious'.
'i'+'ngenious'='INGENIOUS'
(Other definitions for ingenious that I've seen before include "Showing inventiveness" , "Displaying art" , "Artless" , "Skilfully contrived" , "Brilliant" .)