One tiny bit of laudanum will finally cure the unending sickness (3,6)
I believe the answer is:
ill health
'sickness' is the definition.
(I've seen this before)
'one tiny bit of laudanum will finally cure the unending' is the wordplay.
'one' becomes 'i' (Roman numeral).
'tiny bit of' says to take the initial letters.
'will finally' becomes 'l' (I can't justify this - if you can you should give a lot more credence to this answer).
'cure' becomes 'heal' ('heal' can be a synonym of 'cure').
'unending' means to remove the last letter.
The initial letter of 'laudanum' is 'l'.
'the' with its final letter removed is 'th'.
'i'+'l'+'l'+'heal'+'th'='ILL-HEALTH'
Can you help me to learn more?
(Other definitions for ill health that I've seen before include "Sickness" , "Poor physical condition" , "Unfitness" , "being far from well" .)