Sister some Unitarian Universalist (3)
I believe the answer is:
nun
'sister' is the definition.
(sister is a term of address for a nun)
'some unitarian universalist' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'NUN' is hidden within 'unitarian universalist'.
(Other definitions for nun that I've seen before include "She's a member of a religious community" , "Letter" , "A palindromic holy woman" , "Black and white pigeon" , "Member of convent" .)