Jack with bird jaw (4)
I believe the answer is:
jowl
'jaw' is the definition.
(I know that jowl is a type of jaw)
'jack with bird' is the wordplay.
'jack' becomes 'J' (playing card abbreviation).
'with' means one lot of letters go next to another.
'bird' becomes 'owl' (owl is a kind of bird).
'j'+'owl'='JOWL'
(Other definitions for jowl that I've seen before include "Jaw - cheek" , "Flesh hanging from the lower jaw" , "Dewlap" , "Fleshy cheek" , "Pendulous double chin" .)