One-way passage (5)
I believe the answer is:
valve
'passage' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'one-way' is the wordplay.
I cannot really understand how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is found within the answer.
'way' could be 'e' (Compass direction - East) and 'e' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for valve that I've seen before include "Flow regulator" , "Device regulating flow" , "Structure with a flap to ensure one-way flow" , "Something to regulate flow" , "Part of brass instrument" .)