Illness in the mind (13)
I believe the answer is:
indisposition
'illness' is the definition.
(I know that indisposition is a type of illness)
'in the mind' is the wordplay.
I cannot quite understand how this works, but
'in' is within the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for indisposition that I've seen before include "Slight illness" , "Disinclination" , "State of being unwell or unfit" .)