A fast runner – or does one hear different? (4)
I believe the answer is:
hare
'a fast runner' is the definition.
(I've seen this before)
'hear different?' is the wordplay.
'different?' is an anagram indicator (I've seen 'different' mean this).
'hear' is an anagram of 'HARE'.
'or does one' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for hare that I've seen before include "Long-eared mammal" , "Swift rodent" , "Rabbit-like creature" , "Animal like large rabbit - let it sit" , "Fast-running mammal" .)