Something new in book, extremely trendy (7)
I believe the answer is:
novelty
'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'.
'novel'+'ty'='NOVELTY'
'in' is the link.
(Other definitions for novelty that I've seen before include "Originality" , "Original item" , "Middle-Eastern language" , "prompting curiosity" , "Freshness" .)