It's lucky the animal is in that end (6)
I believe the answer is:
amulet
'end' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'it's lucky the animal is in' is the wordplay.
'it's lucky' becomes 'at' (I am not sure about this - if you are sure you should believe this answer much more).
'the animal' becomes 'mule' (offspring of a donkey and a horse).
'is in' is an insertion indicator.
'at' enclosing 'mule' is 'AMULET'.
'that' is the link.
Can you help me to learn more?
(Other definitions for amulet that I've seen before include "Mascot" , "Charm to protect from evil" , "Ornament worn as a charm against evil" , "that's charming" , "Lucky trinket" .)