Large grey rain cloud (6)
I believe the answer is:
nimbus
'large grey rain cloud' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nimbus that I've seen before include "Bums in (anag)" , "showers under it" , "Dark, rain-bearing cloud" , "Is numb (anag)" , "Large rain cloud; halo" .)