Characters occasionally dropping in on our Nan, near my sister's house (7)
I believe the answer is:
nunnery
'house' is the definition.
(nunnery is a kind of house)
'characters occasionally dropping in on our nan near my sister's' is the wordplay.
I cannot quite understand how this works, but
'sister' could be 'nun' (sister is a term of address for a nun) and 'nun' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for nunnery that I've seen before include "Convent" , "Hamlet to Ophelia: 'Get thee to a . . . . . . .'" , "Religious establishment" , "Religious house for females" , "sisters' place" .)