A: We are not speaking 
of a stand-alone TSR-clock (which is a
different task), but considering a clock that continuously displays
the time in some part of the output screen of your Turbo Pascal
program.
    You might first want to read the earlier items about ReadKey
usages if you are not familiar with it (you probably are, because
you would not pose this advanced question if you were a novice). 
The
items are the unlikely "How do I disable or capture the break 
key in
Turbo Pascal?" and "How can I read input without 
echoing to the
screen?"
   The general idea is to make the body of the program a repeat
until loop using ReadKey for input and updating the clock display
at suitable junctions within the loop. The scheme is thus something
like the following.
  procedure showtime;
    begin
      { if the second has changed, write the time }
    end;
  :
  repeat
    { do whatever }
    showtime;
    if KeyPressed then
      case ReadKey of
        { whatever }
        { exit rules }
      end;
    showtime;
    :
    showtime;
  until false;
   One trick of the trade is that you must not update your clock
each time the clock routine is encountered. You should test if the
second has changed, and update only then. Else you are liable to 
get
an annoying flicker in your clock.