Bird crest, bit on its head (9)
I believe the answer is:
partridge
'bird' is the definition.
(partridge is a kind of bird)
'crest bit on its head' is the wordplay.
I cannot really understand how this works, but
'crest' could be 'ridge' (geographical term) and 'ridge' is found within the answer.
'bit' could be 'part' (both can mean a piece of something) and 'part' is located in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for partridge that I've seen before include "Flier" , "Game bird (in a pear tree?)" , "1st" , "early Christmas present" , "Animal" .)