Shock obtained from reversing extractor fans (4)
I believe the answer is:
afro
'shock' is the definition.
(shock can mean a mass of hair)
'obtained from reversing extractor fans' is the wordplay.
'reversing' says the letters should be written backwards.
'obtained from' indicates the answer is hidden within the clue.
'orfa' is hidden in the letters of 'extractor fans'.
'orfa' written backwards gives 'AFRO'.
(Other definitions for afro that I've seen before include "Hairstyle with tight curls" , "Big hairstyle" , "Luxuriant (black) hairstyle" , "Styling done to hair" , "Bushy hairstyle" .)