Valve's copper ring available for sale, conceivably? (8)
I believe the answer is:
stopcock
'valve's' is the definition.
(type of valve)
'copper ring available for sale conceivably?' is the wordplay.
I cannot quite see how this works, but
'copper' could be 'pc' (police constable) and 'pc' is located in the answer.
'ring' could be 'o' (letter resembling a ring) and 'o' is found within the answer.
'for' could be 'to' (eg both can mean 'in the opinion of') and 'to' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for stopcock that I've seen before include "water regulator" , "Regulator of flow" , "Isolating valve in pipework" , "Valve in water supply" , "Control of flow in pipe" .)