The most common and most likely answer to this crossword clue is the 11 letter word COCKLESHELL
I've seen this crossword clue in The Times
C
O
C
K
L
E
S
H
E
L
L
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 = cockleshell.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 COCKLESHELL mean?
COCKLESHELL noun- The shell of a cockle (or similar shell).
- A small, flimsy boat.