Obsession of bird turning North to sleep soundly (5)
I believe the answer is:
craze
'obsession' 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 craze that I've seen before include "Fashion, fad; crack" , "Fine crack" , "passing fashion" , "passion" , "Widespread fad" .)