Long sentence poet's always put in part of book title (4,7)
I believe the answer is:
life peerage
'title' is the definition.
(title or rank given to a peer)
'long sentence poet's always put in part of book' is the wordplay.
'long sentence' becomes 'life' (life in prison).
'poet's always' becomes 'e'er' ('e'er' is a poetic term for 'ever' or 'always').
'put in' is an insertion indicator.
'part of book' becomes 'page' (a page in a book).
'eer' placed within 'page' is 'peerage'.
'life'+'peerage'='LIFE PEERAGE'
(Another definition for life peerage that I've seen is " Title that is non-hereditary".)