Opera text party animal's written up (8)
I believe the answer is:
libretto
'opera text' is the definition.
(I've seen this in another clue)
'party animal's written up' is the wordplay.
'party' becomes 'lib' (abbreviation for Liberal, uk political party).
'animal' becomes 'otter' (I've seen this before).
'written up' says the letters should be written in reverse (in a down clues, letters go up).
'otter' back-to-front is 'retto'.
'lib'+'retto'='LIBRETTO'
(Other definitions for libretto that I've seen before include "Text written for an opera" , "Opera's words" , "text for musicians" , "Operatic text" , "textbook" .)