Sister's outfit? (5)
I believe the answer is:
habit
'sister's outfit' is the definition.
(a nun's habit - nuns are called sisters)
This is all the clue.
(Other definitions for habit that I've seen before include "Custom or monk's robe" , "Ingrained behaviour" , "Custom, nun's wear" , "Regular practice" , "What might one have a regular tendency to" .)