Sister in barn undressed (3)
I believe the answer is:
nun
'sister' is the definition.
(sister is a term of address for a nun)
'in barn undressed' is the wordplay.
'in' indicates the answer is hidden within the clue.
'NUN' can be found hidden inside 'barn undressed'.
(Other definitions for nun that I've seen before include "Letter" , "Priory female" , "Domestic fancy pigeon" , "Religious woman, a palindrome" , "Religious sister" .)