Poet remains active when book released (7)
I believe the answer is:
emerson
'poet' is the definition.
(US writer Ralph Waldo Emerson)
'remains active when book released' is the wordplay.
I cannot really understand how this works, but
'active' could be 'on' (I've seen this in other clues) and 'on' is found in the answer.
'when' could be 'e' (I've seen this in other clues) and 'e' is present in the answer.
This explanation may well be incorrect...
(Other definitions for emerson that I've seen before include "Ralph Waldo --, US author" , "Ralph Waldo --, US poet" , "Individualist" , "--, Lake and Palmer, rock group" , "Ralph Waldo . . ., 19th century American philosopher and poet" .)