Messenger from heaven descending on a girl (6)
I believe the answer is:
angela
'messenger from heaven' is the definition.
(girl's name)
'descending on a girl' is the wordplay.
'descending on' says to put letters next to each other (in a down clue, some letters go on top of others).
'girl' becomes 'angel' ().
'a' after 'angel' is 'ANGELA'.
(Other definitions for angela that I've seen before include "------ Gheorghiu, diva" , "girl" , "woman" , ". . . Lansbury (actress)" , "- Gheorghiu, Romanian soprano" .)