Crossword Genius

500 ahead, let's assume (3)

Ross

I believe the answer is:

don

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

'assume' is the definition.
(I know that assume can be written as don)

'500 ahead let's' is the wordplay.
I cannot quite understand how this works, but
'500' could be 'd' (Roman numeral for 500) and 'd' is located in the answer.
'ahead' could be 'on' (synonyms) and 'on' is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for don that I've seen before include "Professor" , "European river" , "University teacher" , "Spaniard in a University?" , "Put on the learned man" .)

I've seen this clue in the Evening Standard.
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