The most common and most likely answer to this crossword clue is the 11 letter word HALF-HUNTER
I've seen this crossword clue in The Times
H
A
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Buddy explains!
The straight part
We know that "Watch" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case watch = half-hunter.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.
Crossword Answer definitions
HALF-HUNTER noun- A pocket watch of the hunter style, but with a glass panel or hole in the centre of the lid, giving a view of the hands even when the watch is closed.