Phone feature appeared right before app opening (6)
I believe the answer is:
camera
'phone feature' is the definition.
(I've seen this in another clue)
'appeared right before app opening' is the wordplay.
'appeared' becomes 'came' (I've seen this in other clues).
'right' becomes 'r' (common abbreviation).
'before' is a charade indicator (letters next to each other).
'opening' says to take the initial letters.
The initial letter of 'app' is 'a'.
'came'+'r'+'a'='CAMERA'
(Other definitions for camera that I've seen before include "Way of making pictures" , "An image-capturing device" , "Device for taking photos" , "Brownie possibly" , "Device for taking photographs" .)