Bright idea, heading off in downpour? (9)
I believe the answer is:
rainstorm
'downpour?' is the definition.
(I have seen 'Heavy downpour' mean 'rainstorm' so perhaps 'downpour' could also mean 'rainstorm')
'bright idea heading off' is the wordplay.
'bright idea' becomes 'brainstorm' (I've seen this before).
'heading off' means to remove the first letter.
'brainstorm' with its initial letter taken away is 'RAINSTORM'.
'in' is the link.
(Other definitions for rainstorm that I've seen before include "squall" , "bad weather" , "Heavy downpour" , "Wild weather" , "Meteorological phenomenon" .)