Old bird plumber has appointment to see? (6)
I believe the answer is:
boiler
'old bird' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for boiler that I've seen before include "Steam-producer" , "tough old bird" , "Container for heating water or a tough fowl" , "Tank for heating water" , "Container in which water is heated" .)