Rain-bearing cloud (6)
I believe the answer is:
nimbus
'rain-bearing cloud' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for nimbus that I've seen before include "Ouse/Trent estuary" , "Dark, rain-bearing cloud" , "showers under it" , "Is numb (anag)" , "One may be seeded" .)