Mental aberration dismissing start of balmy weather event (9)
I believe the answer is:
rainstorm
'balmy weather event' is the definition.
'mental aberration dismissing start' is the wordplay.
'mental aberration' becomes 'brainstorm'.
'dismissing start' means to remove the first letter.
'brainstorm' with its first letter removed is 'RAINSTORM'.
'of' is the link.
(Other definitions for rainstorm that I've seen before include "bad weather" , "squall" , "In bad fall" , "Heavy downpour" , "Wild weather" .)