Banned substance ultimately recommended to runner (4)
I believe the answer is:
drug
'banned substance' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'ultimately recommended to runner' is the wordplay.
'ultimately' suggests the final letters.
'to runner' becomes 'rug' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The final letter of 'recommended' is 'd'.
'd'+'rug'='DRUG'
Can you help me to learn more?
(Other definitions for drug that I've seen before include "Chemical medicament" , "Perhaps an upper" , "Sedate" , "Item of medicine or a narcotic" , "Maybe addictive substance" .)