Expert air elevated one's forecast (9)
I believe the answer is:
prognosis
'forecast' is the definition.
('prognosis' can be a synonym of 'forecast')
'expert air elevated one's' is the wordplay.
'expert' becomes 'pro' (I've seen this before).
'air' becomes 'song' (I've seen this before**).
'elevated' is a reversal indicator.
'one's' becomes 'is' (roman numeral).
'song' in reverse letter order is 'gnos'.
'pro'+'gnos'+'is'='PROGNOSIS'
(Other definitions for prognosis that I've seen before include "Medical opinion" , "Forecast of course of illness as by doctor" , "Prediction as to how something will turn out" , "Forecast, outlook" , "Medical prediction" .)