Bird that is quietly perched on top (3)
I believe the answer is:
pie
'bird' is the definition.
(sort of bird)
'that is quietly perched on top' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'quietly' becomes 'p' (musical symbol).
'perched on top' is a charade indicator (letters next to each other) (in a down clue, some letters go above others).
'ie' after 'p' is 'PIE'.
(Other definitions for pie that I've seen before include "Pasty - former Indian coin - mixed type - bird" , "In the sky, this is wishful thinking" , "what to cook for dinner?" , "Pastry product" , "Irrational things to do with circles and logs" .)