Introduction to Leeds United fanzine stirred up disorder (9)
I believe the answer is:
influenza
'disorder' is the definition.
(influenza is a kind of health problem)
'introduction to leeds united fanzine stirred up' is the wordplay.
'introduction to' suggests taking the first letters.
'stirred up' indicates anagramming the letters (letters get stirred up or mixed).
The first letters of 'leeds united' is 'lu'.
'lu'+'fanzine'='lufanzine'
'lufanzine' anagrammed gives 'INFLUENZA'.
(Other definitions for influenza that I've seen before include "respiratory ailment" , "Highly infectious viral disease" , "Respiratory complaint" , "Nasty viral disease, usually of winter" , "Winter virus disease, even birds get it" .)