Readers insult a terrible book (8,6)
I believe the answer is:
treasure island
'book' is the definition.
(novel by Robert Louis Stevenson)
'readers insult a terrible' is the wordplay.
'terrible' is an anagram indicator.
'readers'+'insult'+'a'='readersinsulta'
'readersinsulta' is an anagram of 'TREASURE ISLAND'.
(Other definitions for treasure island that I've seen before include "Tale of adventure" , "Book by Stevenson" , "RLS novel" , "Robert Louis Stevenson novel" , "Classic adventure story" .)