Decrease in expected wind level (7)
I believe the answer is:
dwindle
'decrease' is the definition.
(dwindling is a kind of decreasing)
'in expected wind level' is the wordplay.
'in' says the answer is hidden in the clue.
'DWINDLE' can be found hidden inside 'expected wind level'.
(Other definitions for dwindle that I've seen before include "Flag" , "become weaker" , "'Die down, grow less (7)'" , "Become smaller or fewer" , "tail off" .)