Crossword Genius

Rather old to be harnessing a horse? (7,2,1,3)

Ross

I believe the answer is:

getting on a bit

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

'rather old' is the definition.
(I've seen this before)

'be harnessing a horse?' is the wordplay.
I cannot really understand how this works, but
'be' could be 'go' (going is a kind of being) and 'go' is found within the answer.
'a' is found in the answer.
This explanation may well be incorrect...

'to' acts as a link.

Can you help me to learn more?

(Another definition for getting on a bit that I've seen is " Rather old".)

I've seen this clue in The Globe and Mail and The Guardian.
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