Animal, bag finally put on back (4)
I believe the answer is:
frog
'animal' is the definition.
(frog is a kind of animal)
'bag finally put on back' is the wordplay.
'finally' indicates one should take the final letters.
'put on' means one lot of letters go next to another.
'back' becomes 'fro' ('fro' can be a synonym of 'back'**).
The last letter of 'bag' is 'g'.
'g' put after 'fro' is 'FROG'.
(Other definitions for frog that I've seen before include "Amphibious jumper" , "Grown-up tadpole" , "eg Kermit" , "Amphibious neighbour of 12?" , "Common amphibian" .)