Cooling device (3)
'cooling device' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for fan that I've seen before include "Keen follower; cooler" , "Great enthusiast" , "Manually wielded cooling device" , "Cooling device for enthusiastic supporter" , "Enthusiastic admirer as of pop star" .)
(Other definitions for air conditioner that I've seen before include "Ventilation system" , "Building atmosphere controller" , "Cooler" .)