Sister in conundrum (3)
I believe the answer is:
nun
'sister' is the definition.
(sister is a term of address for a nun)
'in conundrum' is the wordplay.
'in' indicates the answer is hidden within the clue.
'NUN' is hidden within 'conundrum'.
(Other definitions for nun that I've seen before include "religious person" , "One in holy order" , "Black and white pigeon" , "Holy sister" , "Woman respecting order" .)