Label attached to the back of this animal (4)
I believe the answer is:
stag
'animal' is the definition.
(stag is a kind of animal)
'label attached to the back of this' is the wordplay.
'label' becomes 'tag' ('tag' can be a synonym of 'label'**).
'attached to' says to put letters next to each other.
'the back of' suggests the final letters.
The final letter of 'this' is 's'.
'tag' after 's' is 'STAG'.
(Other definitions for stag that I've seen before include "One who speculates" , "speculator wanting quick profit" , "one may get in a rut" , "Stock Exchange speculator standing" , "without female escort" .)