Bird's crest (a bit on top) (9)
I believe the answer is:
'bird's' is the definition.
(partridge is a kind of bird)
'crest a bit on top' is the wordplay.
'crest' becomes 'ridge' (I've seen this before).
'a bit' becomes 'part' (I've seen this in other clues).
'on top' means one lot of letters go next to another (down clues only).
'ridge' after 'part' is 'PARTRIDGE'.
(Other definitions for partridge that I've seen before include "Game bird (in a pear tree?)" , "Flier" , "Peculiarly apt girder for the game bird" , "Animal" , "Flighty type" .)