Far out old hairstyle (4)
I believe the answer is:
afro
'hairstyle' is the definition.
(afro is a kind of hairstyle)
'far out old' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'far' with letters rearranged gives 'afr'.
'afr'+'o'='AFRO'
(Other definitions for afro that I've seen before include "One cut" , "configuration of locks" , "Hairstyle with tight curls" , "Big curly hairdo" , "Style of haircut" .)