Mental, worrying about one complaint (7)
I believe the answer is:
ailment
'complaint' is the definition.
(I know that complaint can be written as ailment)
'mental worrying about one' is the wordplay.
'worrying' indicates an anagram.
'about' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'mental' anagrammed gives 'alment'.
'alment' placed around 'i' is 'AILMENT'.
(Other definitions for ailment that I've seen before include "Tummy upset, for instance" , "Tim Alen has an odd indisposition." , "Slight but persistent illness" , "infirmity" , "problem" .)