Bird with part missing - crest? (5)
I believe the answer is:
ridge
'crest?' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ridge that I've seen before include "Raised earth between furrows" , "Narrow hilltop range" , "watershed ..." , "Line of higher ground" , "Line where two upward sloping surfaces meet" .)