Start unseemly riot around noon (5)
I believe the answer is:
intro
'start' is the definition.
(I've seen this before)
'unseemly riot around noon' is the wordplay.
'unseemly' is an anagram indicator.
'around' is an insertion indicator.
'noon' becomes 'n'.
'riot' is an anagram of 'itro'.
'itro' going around 'n' is 'INTRO'.
(Other definitions for intro that I've seen before include "(Short) section before Chapter 1, say" , "Prelude" , "One starts" , "Lead" , "Beginning (informal)" .)