Getting on a treeline alongside yard? (7)

I believe the answer is:

elderly

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

'getting on' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for elderly that I've seen before include "Old or ageing" , "Fairly old, past middle age" , "Past middle age, a senior citizen" , "Silver" , "Long in the tooth" .)

I've seen this clue in the Sydney Morning Herald.

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