Party animal beginning to riot, say (5)
I believe the answer is:
raver
'party animal' is the definition.
(I've seen this before)
'beginning to riot say' is the wordplay.
'beginning to' indicates taking the first letters.
'say' becomes 'aver' ('aver' can be a synonym of 'say').
The initial letter of 'riot' is 'r'.
'r'+'aver'='RAVER'
(Other definitions for raver that I've seen before include "Madman" , "Dedicated partygoer" , "one talking incoherently" , "Uninhibited person" , "Wild partygoer" .)