Crossword Genius

Unit equal to inverse of time divided by speed of light (4)

Ross

I believe the answer is:

acre

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

'unit' is the definition.
(acre is a kind of unit)

'equal to inverse of time divided by speed of light' is the wordplay.
I cannot quite understand how this works, but
'speed of light' could be 'c' (as in E=mc2) and 'c' is found in the answer.
'equal' could be 'are' (I've seen this in another clue) and 'are' is present in the remaining letters.
No letters remain.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for acre that I've seen before include "Care about the piece of land" , "Imperial land measurement" , "Area measure - Israeli port - Brazilian state" , "Imperial unit of area" , "marine location in Israel" .)

I've seen this clue in The Telegraph.
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