What is this?

[lepetitmartien]

Don't dream too much, if it's the only leaf, and there's this kind of damage, I guess it won't survive.

Certainly something fishy round the roots or a weather like condition issue (cold/wet).

You can always try, but cut that leaf, and check the roots ! If it is strong enough to recover and it'll be long if any chance of, it won't if it's rotting inside the pot.