Sister welcomed by brethren, unsisterly (3)
I believe the answer is:
nun
'sister' is the definition.
(sister is a term of address for a nun)
'welcomed by brethren unsisterly' is the wordplay.
'welcomed by' says the answer is hidden in the clue.
'NUN' is hidden in the letters of 'brethren unsisterly'.
(Other definitions for nun that I've seen before include "Letter" , "nuisance in pine forest" , "Woman in religious order" , "She's a member of a religious community" , "Sister of mercy" .)