Long-winded poem swallows half of book (7)
I believe the answer is:
verbose
'long-winded' is the definition.
('verbose' can be a synonym of 'long-winded')
'poem swallows half of book' is the wordplay.
'poem' becomes 'verse' (verse is a kind of poem).
'swallows' is an insertion indicator.
'half of' indicates taking half.
'book' halved is 'bo'.
'verse' placed around 'bo' is 'VERBOSE'.
(Other definitions for verbose that I've seen before include "Circumlocutory" , "Loquacious" , "'Long-winded, wordy (7)'" , "Wordy, prolix" , "Observe (anag) -- long-winded" .)