Careless but lenient confusion with horse (9)
I believe the answer is:
'careless' is the definition.
(associated in meaning)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for negligent that I've seen before include "Not taking due care", "Careless, remiss", "Lax", "Failing to take due care", "Of the study of insects".)
I've seen this clue in Sydney Morning Herald.