Unfinished Harper Lee novel about months passing (9)
I believe the answer is:
ephemeral
'passing' is the definition.
('ephemeral' can be a synonym of 'passing')
'unfinished harper lee novel about months' is the wordplay.
'unfinished' means to remove the last letter.
'novel' indicates an anagram.
'about' indicates putting letters inside.
'months' becomes 'm'.
'harper' with its last letter taken away is 'harpe'.
'harpe'+'lee'='harpelee'
'harpelee' is an anagram of 'epheeral'.
'epheeral' enclosing 'm' is 'EPHEMERAL'.
(Other definitions for ephemeral that I've seen before include "just for a day?" , "Short lived" , "life's too short" , "A mere help (anag) - short-lived" , "Transient, fleeting" .)