Disorder, not initially serious, developed (8)
I believe the answer is:
neurosis
'disorder' is the definition.
(I have seen 'Mental disorder' mean 'neurosis' so perhaps 'disorder' could also mean 'neurosis')
'not initially serious developed' is the wordplay.
'initially' suggests taking the first letters.
'developed' indicates anagramming the letters.
The initial letter of 'not' is 'n'.
'serious' is an anagram of 'eurosis'.
'n'+'eurosis'='NEUROSIS'
(Other definitions for neurosis that I've seen before include "Mental disorder of resinous origin" , "Mild mental problem" , "Mild mental condition" , "Resinous (anag) -- unspecific personality disturbance" , "Mild condition such as obsessive behaviour" .)