Topless omnibus, dreadful in rain cloud (6)
I believe the answer is:
nimbus
'rain cloud' is the definition.
(I know that rain cloud can be written as nimbus)
'topless omnibus dreadful' is the wordplay.
'topless' means to remove the first letter.
'dreadful' indicates anagramming the letters.
'omnibus' with its first letter taken away is 'mnibus'.
'mnibus' anagrammed gives 'NIMBUS'.
'in' acts as a link.
(Other definitions for nimbus that I've seen before include "A black rain cloud" , "Type of rain cloud" , "Storm cloud" , "round one's head?" , "it could bring rain" .)