What has some fail — mental illness (7)
I believe the answer is:
ailment
'what has' is the definition.
I can't tell whether this definition defines the answer.
'some fail mental illness' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'AILMENT' is hidden within 'fail mental illness'.
Can you help me to learn more?
(Other definitions for ailment that I've seen before include "Bodily disorder or disease" , "Complaint, affliction" , "Malady" , "Tim Alen has an odd indisposition." , "Minor illness" .)