Old nun with guise terribly naive (9)
I believe the answer is:
ingenuous
'naive' is the definition.
(similar in meaning)
'old nun with guise terribly' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'with' says to put letters next to each other.
'terribly' indicates anagramming the letters.
'o'+'nun'='onun'
'onun' put after 'guise' is 'guiseonun'.
'guiseonun' is an anagram of 'INGENUOUS'.
(Other definitions for ingenuous that I've seen before include "Free from deception" , "Naive" , "lacking sophistication" , "Innocent, unsuspecting" , "Frank" .)