Crossword Genius

Trees will not usually grow above this height (6,4)

Ross

I believe the answer is:

timber line

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

'trees will not usually grow above this height' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

This is the entire clue.

Can you help me to learn more?

(Other definitions for timber line that I've seen before include "Treetops mark it" , "Far reach of wood" , "woodland boundary" .)

I've seen this clue in The Globe and Mail.
Want a hint initially instead of a full solution? Install my app