I will cure half of this sickness (3,6)
I believe the answer is:
ill health
'sickness' is the definition.
(I've seen this before)
'i will cure half of this' is the wordplay.
'i will' becomes 'ill' (I've seen this before).
'cure' becomes 'heal' ('heal' can be a synonym of 'cure').
'half of' indicates taking half.
'this' with half the letters taken is 'th'.
'ill'+'heal'+'th'='ILL-HEALTH'
(Other definitions for ill health that I've seen before include "Sickness" , "Unfitness" , "Poor physical condition" , "being far from well" .)