Fanatical about an ascendant singer (7)
I believe the answer is:
madonna
'singer' is the definition.
(American singer)
'fanatical about an ascendant' is the wordplay.
'fanatical' becomes 'mad' (I've seen this in another clue).
'about' becomes 'on' (as in a book 'on' a certain topic).
'ascendant' says the letters should be written backwards (in a down clue, going upwards).
'an' written backwards gives 'na'.
'mad'+'on'+'na'='MADONNA'
(Other definitions for madonna that I've seen before include "Religious work" , "Picture or statue of the Virgin Mary" , "Like a Virgin singer" , "religious painting" , "American pop singer" .)