The Timing Clock message (like all messages in the F8…FF range) is a System Real-Time Message.
The specification says:
Real Time messages can be sent at any time and may be inserted anywhere in a MIDI data stream, including between Status and Data bytes of any other MIDI messages. Giving Real-Time messages high priority allows synchronization to be maintained while other operations are being carried out.
Real time messages may be inserted between data bytes of an Exclusive message in order to maintain synchronization, and can not be used to terminate an exclusive message