'Potter' bilge holds up book (8)
I believe the answer is:
libretto
'book' is the definition.
(libretto is a kind of book)
'potter bilge holds up' is the wordplay.
'holds' indicates the answer is hidden within the clue.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'otterbil' is hidden within 'potter bilge'.
'otterbil' backwards is 'LIBRETTO'.
(Other definitions for libretto that I've seen before include "The words of an opera or musical play" , "Book of words of an opera" , "Operatic book" , "text for musicians" , "11 down book" .)