Do nothing about contrary men (4)
I believe the answer is:
afro
'do' is the definition.
(type of hairdo)
'nothing about contrary men' is the wordplay.
'nothing' becomes 'FA' (slang term - sweet Fanny Adams).
'about' says the letters should be written backwards.
'contrary' shows that the letters should be reversed in order.
'men' becomes 'OR' (military abbreviation for Other Ranks).
'fa' reversed gives 'af'.
'or' written backwards gives 'ro'.
'af'+'ro'='AFRO'
(Other definitions for afro that I've seen before include "Big hairstyle" , "Style of haircut" , "Curly hairstyle" , "Of hair style, tightly curled and bushy" , "Hair in tight curls" .)