Part of book removed from a theatregoer, perhaps? (8)
I believe the answer is:
I believe this is a double definition.
'part of book' is the first definition.
(I've seen this before)
'removed from a theatregoer perhaps?' is the second definition.
(as in an operating theatre)
(Other definitions for appendix that I've seen before include "extra pages" , "Final part of work" , "sometimes removed?" , "often discarded in theatre" , "Outgrowth of intestine, often surgically removed" .)