Main problem with a disorder (5)
I believe the answer is:
mania
'disorder' is the definition.
(mania is a kind of disorder)
'main problem with a' is the wordplay.
'problem with' indicates anagramming the letters.
'main' mixed up with 'a' makes 'MANIA'.
(Other definitions for mania that I've seen before include "Violent madness or extreme enthusiasm" , "thing" , "wildness" , "Madness - craze" , "Insanity" .)