Corpulent swimmer taking short walk at reduced speed (6)
I believe the answer is:
walrus
'corpulent swimmer' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'short walk at reduced speed' is the wordplay.
'short' means to remove the last letter.
'at' means one lot of letters go next to another.
'reduced speed' becomes 'rus' (I can't explain this - if you can you should give a lot more credence to this answer).
'walk' with its final letter taken away is 'wal'.
'wal'+'rus'='WALRUS'
'taking' acts as a link.
Can you help me to learn more?
(Other definitions for walrus that I've seen before include "Kind of heavy moustache that hangs down at sides" , "Carpenter's companion?" , "Marine mammal with tusks and a tough hide" , "Sea beast with 6" , "Marine mammal with two large tusks" .)