Refuse to allow tailless domesticated fowl (6)
I believe the answer is:
bantam
'fowl' is the definition.
(bantam is a kind of fowl)
'refuse to allow tailless domesticated' is the wordplay.
'refuse to allow' becomes 'ban' (to ban is to forbid).
'tailless' means to remove the last letter (the tail of the word is removed).
'domesticated' becomes 'tame' (similar in meaning).
'tame' with its final letter taken off is 'tam'.
'ban'+'tam'='BANTAM'
(Other definitions for bantam that I've seen before include "aggressive little sort" , "Boxing weight or domestic fowl" , "A cock; one small, aggressive" , "Small chicken and light boxer" , "Small chicken - small soldier" .)