Introduction to Leeds United fanzine stirred up disorder (9)
I believe the answer is:
influenza
'disorder' is the definition.
(influenza is a kind of disease)
'introduction to leeds united fanzine stirred up' is the wordplay.
'introduction to' indicates taking the first letters.
'stirred up' indicates an anagram (letters get stirred up or mixed).
The initial letters of 'leeds united' is 'lu'.
'lu'+'fanzine'='lufanzine'
'lufanzine' with letters rearranged 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" .)