Okay! So when defining cron handlers, you cannot nest them:
=========================================================
#!
package require cron

# Done timer
::cron::in done 20 {incr ::done}

# Ten second timer
::cron::in ten_seconds 10 {
    puts "10 second timer triggered!"
    # Nested timer
    ::cron::in five_seconds 5 {
        puts "5 seconds triggered!"
    }
}

vwait ::done
=========================================================
Gives us this output:
=========================================================
second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
10 second timer triggered!
5 seconds triggered!
~]$
=========================================================

If you instead turn the inner cron statement to an after Xms y, it works as expected:
=========================================================
#!
package require cron

# Done timer
::cron::in done 20 {incr ::done}

# Ten second timer
::cron::in ten_seconds 10 {
    puts "10 second timer triggered!"
    # Nested timer
    after 5000 {
        puts "After 5 seconds triggered!"
    }
}

vwait ::done
=========================================================

=========================================================
~]$ ./cron.tcl
10 second timer triggered!
After 5 seconds triggered!
~]$
=========================================================