Is there a Real Time Clock in FEZ Spider?

Is there a Real Time Clock in FEZ Spider?

If yes, how can I read and write the RTC?

Yes - all it lacks is a battery, you can use extender module to connect a coin cell battery between ground and pin 5 on socket 2. Then in code can use Microsoft.SPOT.SystemTime like [this].

Any specifik hardware you can recommend to hold a coin-cell in place?

Multiple options available:

http://www.bing.com/images/search?q=coin+cell+holder&FORM=HDRSC2

Depends on your design. There is nothing special about them.