Bird's trick to ascend over mountain feature (9)
I believe the answer is:
partridge
'bird's' is the definition.
(partridge is a kind of bird)
'trick to ascend over mountain feature' is the wordplay.
'trick' becomes 'trap' (I've seen this in another clue).
'to ascend' says the letters should be written in reverse (I've seen 'ascending' mean this).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'mountain feature' becomes 'ridge' (I've seen this in another clue).
'trap' backwards is 'part'.
'part'+'ridge'='PARTRIDGE'
(Other definitions for partridge that I've seen before include "early Christmas present" , "Flighty type" , "Flier" , "Animal" , "1st" .)