In a mad poet, one sees a first-class back - the dope! (6)
I believe the answer is:
opiate
'dope' is the definition.
(I've seen this before)
'in a mad poet one sees a first-class back' is the wordplay.
'in' means one lot of letters goes inside another.
'a mad' indicates an anagram.
'one' becomes 'a' (a thing is one thing).
'sees a first-class back' becomes 'i' (I can't explain this - if you can you should give a lot more credence to this answer).
'poet'+'a'='poeta'
'poeta' with letters rearranged gives 'opate'.
'opate' going around 'i' is 'OPIATE'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for opiate that I've seen before include "Does it ease pain" , "It induces sleep" , "drug to aid rest" , "Narcotic drug" , "Narcotic, such as morphine" .)