Bird’s split crest (9)
I believe the answer is:
partridge
'bird's' is the definition.
(partridge is a kind of bird)
'split crest' is the wordplay.
'split' becomes 'part' (synonyms).
'crest' becomes 'ridge' (I've seen this before).
'part'+'ridge'='PARTRIDGE'
(Other definitions for partridge that I've seen before include "Animal" , "1st" , "Widespread bird" , "early Christmas present" , "Flier" .)