Indisposition of one in mental breakdown (7)
I believe the answer is:
ailment
'indisposition' is the definition.
(I've seen this before)
'one in mental breakdown' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'breakdown' indicates anagramming the letters.
'mental' with letters rearranged gives 'alment'.
'i' inserted into 'alment' is 'AILMENT'.
'of' is the link.
(Other definitions for ailment that I've seen before include "Sickness" , "Tim Alen has an odd indisposition." , "Slight but persistent illness" , "Bodily disorder or disease" , "I lament (anag.)" .)