Disorder is in suit's tailoring (9)
I believe the answer is:
sinusitis
'tailoring' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'disorder is in suit's' is the wordplay.
'disorder' indicates anagramming the letters (a disordered version of the letters).
'is'+'in'+'suits'='isinsuits'
'isinsuits' with letters rearranged gives 'SINUSITIS'.
Can you help me to learn more?
(Other definitions for sinusitis that I've seen before include "Nasal membrane inflammation" , "nasal problem" , "Inflammation in the skull" , "Inflammation of nasal cavities" , "Inflammation of head cavities" .)