Gloomy second novel unfurls endlessly (8)
I believe the answer is:
mournful
'gloomy' is the definition.
(I've seen this before)
'second novel unfurls endlessly' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'novel' is an anagram indicator.
'endlessly' means to remove the last letter.
'unfurls' with its final letter taken away is 'unfurl'.
'unfurl' with letters rearranged gives 'urnful'.
'mo'+'urnful'='MOURNFUL'
(Other definitions for mournful that I've seen before include "Sad, regretful" , "Gloomy" , "Lugubrious" , "Feeling grief" , "Expressing sadness" .)