Prohibition on headgear for fowl (6)
I believe the answer is:
bantam
'for fowl' is the definition.
(bantam is a kind of fowl)
'prohibition on headgear' is the wordplay.
'prohibition' becomes 'ban' (synonyms).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'headgear' becomes 'tam' (tam is a kind of headgear).
'ban'+'tam'='BANTAM'
(Other definitions for bantam that I've seen before include "Small chicken and light boxer" , "Domestic fowl; small fighter" , "Boxing weight or feisty fowl" , "poultry" , "Half of 1D [GAME OR BIRD]" .)