Bird pen fixture with round ridge opening tipped over (5)
I believe the answer is:
robin
'bird' is the definition.
(robin is a kind of bird)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for robin that I've seen before include "Small brown and red bird" , "Red-breasted Christmas card bird" , "Bird on many cards" , "Bird; man's name" , "Seasonal bird" .)