Familiar rascal periodically found in nunnery (10)
I believe the answer is:
conversant
'familiar' is the definition.
(I know that familiar can be written as conversant)
'rascal periodically found in nunnery' is the wordplay.
'periodically' indicates alternate letters (letters must be selected 'every so often').
'found in' indicates putting letters inside.
'nunnery' becomes 'convent' (I've seen this before).
The alternating letters of 'rascal' are 'rsa'.
'rsa' going inside 'convent' is 'CONVERSANT'.
(Other definitions for conversant that I've seen before include "Acquainted" , "Au fait (with)" , "Familiar" .)