Familiar rascal oddly hiding in nunnery (10)
I believe the answer is:
conversant
'familiar' is the definition.
(thesaurus)
'rascal oddly hiding in nunnery' is the wordplay.
'oddly' indicates alternate letters.
'hiding in' means one lot of letters goes inside another.
'nunnery' becomes 'convent' (I've seen this before).
The alternating letters of 'rascal' are 'rsa'.
'rsa' put into 'convent' is 'CONVERSANT'.
(Other definitions for conversant that I've seen before include "Au fait (with)" , "Familiar" , "Acquainted" .)