Decrease in unprecedented wind levels (7)
I believe the answer is:
dwindle
'decrease' is the definition.
(I know that dwindle is a type of decrease)
'in unprecedented wind levels' is the wordplay.
'in' says the answer is hidden in the clue.
'DWINDLE' can be found hidden inside 'unprecedented wind levels'.
(Other definitions for dwindle that I've seen before include "Diminish gradually" , "(Flow) decrease" , "Lessen; fade out" , "Grow less and fade away" , "Grow less - waste away" .)