Drug sent up transport line (9)
I believe the answer is:
substance
'drug' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'sent up transport line' is the wordplay.
'sent up' says the letters should be written backwards.
'transport' becomes 'bus' (bus is a kind of transport).
'line' becomes 'stance' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'bus' reversed gives 'sub'.
'sub'+'stance'='SUBSTANCE'
Can you help me to learn more?
(Other definitions for substance that I've seen before include "power" , "Physical matter" , "Subject matter" , "wealth" , "Matter or material that a thing is made of" .)