Grey lair behind meadow (6)
I believe the answer is:
leaden
'grey' is the definition.
('leaden' can be a synonym of 'grey')
'lair behind meadow' is the wordplay.
'lair' becomes 'den' ('den' can be a synonym of 'lair').
'behind' is a charade indicator (letters next to each other).
'meadow' becomes 'lea' (I've seen this before**).
'den' after 'lea' is 'LEADEN'.
(Other definitions for leaden that I've seen before include "Heavy and slow, as if weighted down with heavy metal" , "Leaned over in slow, heavy way" , "Heavy or slow, like dark-gray metal" , "Dull, heavy, slow" , "Heavy - overcast" .)