Gloomy field retreat (6)
I believe the answer is:
leaden
'gloomy' is the definition.
(thesaurus)
'field retreat' is the wordplay.
'field' becomes 'lea' (I've seen this before).
'retreat' becomes 'den'.
'lea'+'den'='LEADEN'
(Other definitions for leaden that I've seen before include "Slow-moving, being like heavy metal" , "Heavy - overcast" , "Dull, heavy, slow" , "Heavy and slow, as if weighted down with heavy metal" , "Describes grey overcast skies" .)