If the fluid is low, it is probably the clutch slave cylinder.
This is a very easy and pretty inexpensive repair.
Also, this is a very common problem. I think that the slave cylinder is
one of the few weak spots on the Miata.
Some people get 15 or more years out of them, but every Miata that I
have worked on has needed it to be replaced sometime around the 10 year
mark if not earlier.
The clutch slave cylinder on my previous '96M only made it to 60,000
miles, or about 5 years, despite frequent fluid changes starting after
the first 10,700 miles that it on the odometer had when I purchased it.