The asterisk indicates the current terminal session. Suppose if your terminal server is using the line 19, even though u give clear line 19, the line will not be cleared.
Example
Router#clear line 19
% Not allowed to clear current line
5:39:31 PM
Router#sh line
Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns Int
0 CTY - - - - - 0 1 4/214436 -
1 TTY 9600/9600 - - - - - 44 598 2/8 -
2 TTY 9600/9600 - - - - - 43 405 11026/33074 -
3 TTY 9600/9600 - - - - - 30 861 64/201 -
4 TTY 9600/9600 - - - - - 23 279 64/200 -
5 TTY 9600/9600 - - - - - 22 102 56/174 -
6 TTY 9600/9600 - - - - - 16 61 11057/33178 -
7 TTY 9600/9600 - - - - - 19 181 11138/33423 -
8 TTY 9600/9600 - - - - - 16 260 11147/33453 -
9 TTY 9600/9600 - - - - - 23 95638 11142/33438 -
10 TTY 9600/9600 - - - - - 16 141025 68/209 -
11 TTY 9600/9600 - - - - - 5 0 0/0 -
12 TTY 9600/9600 - - - - - 17 786 11101/33307 -
13 TTY 9600/9600 - - - - - 11 123 11141/33428 -
14 TTY 9600/9600 - - - - - 15 24 0/0 -
15 TTY 9600/9600 - - - - - 3 92 47/142 -
16 TTY 9600/9600 - - - - - 5 297 0/0 -
17 AUX 9600/9600 - - - - - 0 0 0/0 -
18 VTY - - - - - 44 0 0/0 -
* 19 VTY - - - - - 23 0 0/0 -
5:39:55 PM
Even after clearing the line used by TermServer will not be cleared