On a drug, coming up (4)
I believe the answer is:
atop
'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" .)