Right to declare one’s a partygoer (5)
I believe the answer is:
raver
'one's a partygoer' is the definition.
(someone who goes to raves)
'right to declare' is the wordplay.
'right' becomes 'r' (common abbreviation).
'to declare' becomes 'aver' (averring is a kind of declaring).
'r'+'aver'='RAVER'
(Other definitions for raver that I've seen before include "Dedicated partygoer" , "Uninhibited person" , "Party animal" , "Wild partygoer" , "Madman" .)