Release missing duck — it follows bear (7)
I believe the answer is:
undergo
'bear' is the definition.
(undergoing is a kind of bearing)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for undergo that I've seen before include "Suffer reverse" , "having to travel by tube" , "Experience, be subjected to" , "Endure, experience something" , "put up with" .)