Can anybody give an example about Clearance Expiry Time?
The current version of the AIM says:


  Clearance Void Times:
  
  A pilot may receive a clearance, when operating from an airport
  without a control tower, which contains a provision for the clearance
  to be void if not airborne by a specific time.
  
  A pilot who does not depart prior to the clearance void time must
  advise ATC as soon as possible of their intentions. ATC will normally
  advise the pilot of the time allotted to notify ATC that the aircraft
  did not depart prior to the clearance void time. This time cannot
  exceed 30 minutes. 
  
  Failure of an aircraft to contact ATC within 30 minutes after the
  clearance void time will result in the aircraft being considered
  overdue and search and rescue procedures initiated.
  
  EXAMPLE− Clearance void if not off by (clearance void time) and, if
  required, if not off by (clearance void time) advise (facility) not
  later than (time) of intentions.


Let's say you're using a GCO or telephone to contact an ARTCC for your clearance out in a low-population area with little infrastructure making it impossible to contact ATC from the ground via standard radio equipment.  After you receive your clearance, you will hang up the call or change frequencies and break contact with ATC.

Due to the break in communication, they will end your IFR clearance by assigning a void time in case you don't get airborne as soon as expected.  If you delay beyond your void time, you would need to call and get a new clearance that you would expect to have issued with a new void time.  The void time is short enough that the implied intention is for the aircraft to receive the clearance while stopped at the runway's hold short line already prepared for takeoff.