Unfinished Harper Lee novel about month passing (9)
I believe the answer is:
ephemeral
'passing' is the definition.
(I know that passing can be written as ephemeral)
'unfinished harper lee novel about month' is the wordplay.
'unfinished' means to remove the last letter.
'novel' indicates an anagram (letters in a new or novel order).
'about' means one lot of letters goes inside another.
'month' becomes 'M' (standard abbreviation).
'harper' with its final letter taken away is 'harpe'.
'harpe'+'lee'='harpelee'
'harpelee' anagrammed gives 'epheeral'.
'epheeral' placed around 'm' is 'EPHEMERAL'.
(Other definitions for ephemeral that I've seen before include "not going to last long" , "Lasting only briefly" , "Short lived" , "Having only brief existence" , "staying for a little while" .)