Gloomy book to exfiltrate for publication (5)
I believe the answer is:
bleak
'gloomy' is the definition.
(bleak means gloomy or sad)
'book to exfiltrate for publication' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'to exfiltrate for publication' becomes 'leak' (to leak information for publication).
'b'+'leak'='BLEAK'
(Other definitions for bleak that I've seen before include "Bare, inhospitable" , "Dreary; cold, miserable" , "Austere" , "Exposed and barren" , "Offering little hope" .)