In due course I see the last of chronic back disorder (5)
I believe the answer is:
chaos
'disorder' is the definition.
(chaos is a kind of disorder)
'in due course i see the last of chronic back' is the wordplay.
'in due course' becomes 'so' (I can't explain this - if you can you should give a lot more credence to this answer).
'i see' becomes 'ah' ('ah' can be used to express understanding).
'the last of' indicates one should take the final letters.
'back' says the letters should be written in reverse.
The final letter of 'chronic' is 'c'.
'so'+'ah'+'c'='soahc'
'soahc' in reverse letter order is 'CHAOS'.
Can you help me to learn more?
(Other definitions for chaos that I've seen before include "Pandemonium" , "confused state" , "Bedlam" , "Totally disordered state" , "Total disorder and confusion" .)