Vanish into thin air? (9)
(Other definitions for dissipate that I've seen before include "Be dispelled/dispersed" , "scatter" , "Exhaust" , "Waste (money/energy)" , "Fritter away" .)
'vanish into thin air?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for evaporate that I've seen before include "Disappear into thin air" , "Turn from liquid to vapour" , "let off steam" , "go a stage further?" , "Dissolve into the air" .)