Careless but lenient confusion with horse (9)
I believe the answer is:
negligent
'careless' is the definition.
(similar in meaning)
I cannot understand how the rest of the clue works.
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" .)