Crossword Genius

On a drug, coming up (4)

Ross

I believe the answer is:

atop

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

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

'on a drug coming' is the wordplay.
'on' becomes 'at' (I've seen this in other clues).
'a drug coming' becomes 'op' (I can't explain this - if you can you should give a lot more credence to this answer).
'at'+'op'='ATOP'

Can you help me to learn more?

(Other definitions for atop that I've seen before include "Positioned on" , "Situated on" , "On (poetic)" , "In superior position" , "on the head" .)

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