The most common and most likely answer to this crossword clue is the 10 letter word RE-ENTRANT
R
E
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E
N
T
R
A
N
T
Buddy explains!
The straight part
We know that "a sort of valley" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a sort of valley = re-entrant.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 RE-ENTRANT mean?
RE-ENTRANT adjective- Reentering; pointing inward.
- (of a mutual exclusion mechanism) Such that the corresponding lock can be reacquired by the locking thread.
- That may be executed more than once at a time either by different threads, or because of recursion.
- (of a part of a rocket) Designed to return to the Earth's atmosphere.
- Of or pertaining to reentry (in the heart)
- (of a ukulele or similar instrument) Having the strings not ordered according to pitch (having a re-entry)