Something new in book, extremely trendy (7)
I believe the answer is:
'something new' is the definition.
(I've seen this before)
'book extremely trendy' is the wordplay.
'book' becomes 'novel' (novel is a kind of book).
'extremely' suggests removing the centre (only the extremes of the word are used).
'trendy' with its centre taken out is 'ty'.
'in' is the link.
(Other definitions for novelty that I've seen before include "Originality" , "Original item" , "Middle-Eastern language" , "prompting curiosity" , "Freshness" .)