She makes an appearance in requiem mass (4)
I believe the answer is:
emma
'she' is the definition.
(I've seen this before)
'makes an appearance in requiem mass' is the wordplay.
'makes an appearance in' indicates the answer is hidden within the clue.
'EMMA' is hidden in the letters of 'requiem mass'.
(Other definitions for emma that I've seen before include "Girl's name - Jane Austen novel" , "novelist's work" , "Miss Woodhouse?" , "Madame Bovary, perhaps" , "companion for Pip" .)