Crossword Genius

Origin of botany? (4)

Ross

I believe the answer is:

root

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

'origin' is the definition.
(I know that origin can be written as root)

'of botany?' is the wordplay.
I cannot quite see how this works, but
'of' could be 'o' and 'o' is located in the answer.
The remaining letters 'rot' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for root that I've seen before include "Reptile" , "Basic source or origin" , "burrow" , "top of tree? Hardly!" , "grub" .)

I've seen this clue in The Mirror.
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