Poet, having released book, remains active (7)
I believe the answer is:
emerson
'poet' is the definition.
(US writer Ralph Waldo Emerson)
'having released book remains active' is the wordplay.
'having released' is a deletion indicator.
'book' becomes 'b' (abbreviation for book).
'remains' becomes 'embers' (remains of a fire).
'active' becomes 'on' (I've seen this in other clues).
'embers' with 'b' taken out is 'emers'.
'emers'+'on'='EMERSON'
(Other definitions for emerson that I've seen before include "--, Lake and Palmer, rock group" , "American author, 1803-82" , "he could write!" , "Individualist" , "American poet and philosopher, d. 1882" .)