It's been quick to jump over the lazy dog (3)
I believe the answer is:
fox
'dog' is the definition.
The definition and answer can be both animals as well as being singular nouns.
Maybe they are linked in a way I don't understand?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for fox that I've seen before include "Puzzle" , "Nonplus" , "Vulpine creature" , "Deceive; Reynard" , "Hunted animal" .)