Mammal, maybe a mule at the end of its tether (5)
I believe the answer is:
lemur
'mammal' is the definition.
(lemur is a kind of mammal)
'a mule at the end of its tether' is the wordplay.
I cannot really see how this works, but
an anagram of 'mule' is 'lemu' which is located in the answer.
A single letter 'r' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'maybe' acts as a link.
Can you help me to learn more?
(Other definitions for lemur that I've seen before include "Nocturnal primate from Madagascar" , "Social climber" , "Arboreal primate native to Madagascar" , "one in the bush?" , "Primate found only in Madagascar" .)