Prohibit headwear for chicken (6)
I believe the answer is:
bantam
'for chicken' is the definition.
(bantam is a kind of chicken)
'prohibit headwear' is the wordplay.
'prohibit' becomes 'ban' (banning is a kind of prohibiting).
'headwear' becomes 'tam' (I've seen this in another clue).
'ban'+'tam'='BANTAM'
(Other definitions for bantam that I've seen before include "Diminutive breed of hen" , "Small breed of chicken" , "Domestic fowl; small fighter" , "Small chicken and light boxer" , "Lightweight boxer and chicken" .)