The most common and most likely answer to this crossword clue is the 10 letter word SHARP-EYED
I've seen this crossword clue in The Times
S
H
A
R
P
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E
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D
Buddy explains!

The straight part
We know that "Keenly observant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case keenly observant = sharp-eyed.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 SHARP-EYED mean?
SHARP-EYED adjective- Having acute vision; having good eyesight.
- Attentive, given to noticing things, aware.