Grey retreat south of meadow (6)
I believe the answer is:
leaden
'grey' is the definition.
(synonyms)
'retreat south of meadow' is the wordplay.
'retreat' becomes 'den' (both can mean a lair or hideout).
'south of' means one lot of letters go next to another (in a down clue, some letters go under others).
'meadow' becomes 'lea' (a lea is a grassy area).
'den' put after 'lea' is 'LEADEN'.
(Other definitions for leaden that I've seen before include "Heavy and slow, as if weighted down with heavy metal" , "Gloomy - slow-moving" , "Describes grey overcast skies" , "Depressing - heavy" , "Leaned over in slow, heavy way" .)