Gloomy nature's in fashion (9)
I believe the answer is:
saturnine
'gloomy' is the definition.
(I've seen this in another clue)
'nature's in fashion' is the wordplay.
'fashion' is an anagram indicator.
'natures'+'in'='naturesin'
'naturesin' is an anagram of 'SATURNINE'.
(Other definitions for saturnine that I've seen before include "dull" , "moody" , "Gloomy (temperament)" , "Turn in sea makes one gloomy" , "Morose looking" .)