Bird's back with practically complete beast (6)
I believe the answer is:
animal
'practically complete beast' is the definition.
'animal' can be an answer for 'beast' ('animal' can be a synonym of 'beast'). I'm not certain of the 'practically complete' bit.
'bird's back' is the wordplay.
'bird' becomes 'ani' (ani is a kind of bird).
'back' becomes 'mal' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ani'+'mal'='ANIMAL'
'with' acts as a link.
Can you help me to learn more?
(Other definitions for animal that I've seen before include "Uncivilised person" , "something in the zoo?" , "Creature having voluntary movement" , "Possibly bear" , "It shares the world with vegetable and mineral" .)