The most common and most likely answer to this crossword clue is the 7 letter word VEDETTE
I've seen this crossword clue in Financial Times
V
E
D
E
T
T
E
More clues leading to the result VEDETTE
Buddy explains!

The straight part
We know that "sentry" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sentry = vedette.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 VEDETTE mean?
VEDETTE noun- A sentinel, usually on horseback, stationed on the outpost of an army, to watch an enemy and give notice of danger.