Melted fuse

[RaymondR]

No idea what it was for, but if the fuse has melted, then it hasn't done its job. It should blow long before it melts.

I've personally experienced this exact scenario where the plastic of the fuse melts before the fuse blows due to constant high current below the limit of the fuse.

 

[Corndoggy]

It is probably suffering from corrosion on the terminals creating a high resistance joint which in turn creates heat, not overloaded hence why the fuse hasn't blown, but any connection that has some form of corrosion, dirt, sand, water ingress will fester into a problem - that's why I avoid driving through mud & water

As said above heat and a melted fuse holder does not mean you have an excessive amount of current and your fuse has not blown.