Heavy and grey (6)
I believe the answer is:
leaden
I believe this clue is a double definition.
'heavy' is the first definition.
(I know that heavy can be written as leaden)
'grey' is the second definition.
(synonyms)
'and' acts as a link.
(Other definitions for leaden that I've seen before include "Slow-moving, being like heavy metal" , "Heavy - plodding" , "Leaned over in slow, heavy way" , "Depressing - heavy" , "Heavy and slow, as if weighted down with heavy metal" .)