The most common and most likely answer to this crossword clue is the 8 letter word STOPCOCK
I've seen this crossword clue in Financial Times
S
T
O
P
C
O
C
K
More clues leading to the result STOPCOCK
- Work colleague initially coming into store for valve
- Old copper in standard valve
- Don’t let bird pipe, controlling emission
- Blow a high note: put in valve
- Valve in vessels raised angle
- . Vessels rise and tip in device to control water supply
- Tap old computer in store
- Valve’s copper ring kept in store
- Valve hit, high-pitched note coming in
- Can it turn off hose carrying C6?
Buddy explains!

The straight part
We know that "Valve" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case valve = stopcock.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does STOPCOCK mean?
STOPCOCK noun- A valve, tap or faucet which regulates the flow of liquid or gas through a pipe.
- A main shutoff for water to a home from a municipal supply. Usually these valves exist in pairs, one outside the property boundary and one inside the property boundary.