Tempests (6)
I believe the answer is:
storms
'tempests' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for storms that I've seen before include "They occur at force 10 or 11 on the Beaufort scale" , "Weather periods when winds are force 11" , "They occur when winds reach force 10" , "Tempests" , "Outbreaks of violent weather" .)