Mammal, a quiet one, chasing the lowest possible temperature (5)
I believe the answer is:
'mammal' is the definition.
(okapi is a kind of mammal)
'a quiet one chasing the lowest possible temperature' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'one' becomes 'I' (Roman numeral).
'chasing' says to put letters next to each other.
'the lowest possible temperature' becomes 'ok' (I can't explain this - if you can you should give a lot more credence to this answer).
'api' after 'ok' is 'OKAPI'.
Can you help me to learn more?
(Other definitions for okapi that I've seen before include "Wild animal" , "Animal of central Africa" , "African animal like giraffe with shorter neck and striped body" , "Ruminant mammal of Africa" , "browser" .)