Ascendant bird is source of magic in primitive religion (7)
I believe the answer is:
animism
'primitive religion' is the definition.
(I've seen this before)
'ascendant bird is source of magic' is the wordplay.
'ascendant' is a reversal indicator (in a down clue, going upwards).
'bird' becomes 'mina' (alternative spelling of myna).
'source of' indicates taking the first letters (the start - or source - of the word).
The initial letter of 'magic' is 'm'.
'mina' reversed gives 'anim'.
'anim'+'is'+'m'='ANIMISM'
'in' acts as a link.
(Other definitions for animism that I've seen before include "primitive religion" , "primitive belief" , "belief system" , "Belief that all natural things have a living soul" , "religious belief" .)