One mental disturbance, an indisposition (7)
I believe the answer is:
ailment
'an indisposition' is the definition.
(I've seen this before)
'one mental disturbance' is the wordplay.
'one' becomes 'I' (Roman numeral).
'disturbance' indicates an anagram.
'i'+'mental'='imental'
'imental' is an anagram of 'AILMENT'.
(Other definitions for ailment that I've seen before include "infirmity" , "Tim Alen has an odd indisposition." , "Tummy upset, for instance" , "Minor illness" , "Physical disorder" .)