Crossword Genius

Where a plane reaches its highest point? (7)

Ross

I believe the answer is:

treetop

I'm a little stuck... Click here to teach me more about this clue!

'where a' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'plane reaches its highest point?' is the wordplay.
I cannot quite understand how this works, but
'plane' could be 'tree' (plane tree is a type of tree) and 'tree' is found in the answer.
'highest' could be 'top' (I've seen this before) and 'top' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for treetop that I've seen before include "Birds sit here" , "High point of a plane, for example" , "To Peter (anag)" , "for baby, a rocky situation" , "Highest branch location" .)

I've seen this clue in The Telegraph.
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