Embargo on mostly tame chicken (6)
I believe the answer is:
bantam
'chicken' is the definition.
(I know that Bantam is a type of chicken)
'embargo on mostly tame' is the wordplay.
'embargo' becomes 'ban' (I've seen this in another clue).
'on' means one lot of letters go next to another.
'mostly' means to remove the last letter.
'tame' with its final letter taken away is 'tam'.
'ban'+'tam'='BANTAM'
(Other definitions for bantam that I've seen before include "A cock; one small, aggressive" , "weight of a pug" , "Domestic fowl; small fighter" , "Little clucker" , "Nab mat (anag)" .)