The most common and most likely answer to this crossword clue is the 6 letter word APICAL
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More clues leading to the result APICAL
Buddy explains!

The straight part
We know that "at the top" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case at the top = apical.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 APICAL mean?
APICAL noun- A sound produced by obstructing the air passage with the tip of the tongue.
- Of or connected with the apex.
- (of a meristem) Situated at the growing tip of the plant or its roots, in comparison with intercalary growth situated between zones of permanent tissue.
- (of a sound) Produced with the tip of the tongue.