How can I get the wall clock time of a running process?

For a running process you can do this:

PID=5462
command ps -p "$PID" -o etime
command ps -p "$PID" --no-headers -o etime

As a general feature you can modify your shell prompt. This is my bash prompt definition:

TERM_RED_START=$'\033[1m\033[31m'
TERM_RED_END=$'\033(B\033[m'
PS1='\nec:$(ec=$?; if [ 0 -eq $ec ];
then printf %-3d $ec;
else echo -n "$TERM_RED_START"; printf %-3d $ec; echo "$TERM_RED_END";
fi) \t  \u@\h:\w\nstart cmd:> '
PS2="cont. cmd:> "

The relevant part for you is the \t for the time.

This does not solve all problems, though. The new prompt will show when the process has ended but the former prompt may have been quite old when the command to be measured was started. So either you remember to renew the prompt before starting long running commands or you have to remember the current time when you want to know how long the current process will have taken.

For a complete solution you need an audit feature (which logs the start and end time of processes). But that may cause a huge amount of data if it cannot be restricted to the shell.

Building upon the accepted answer, you can also continuously monitor elapsed time of a running process using watch:

PID=5462
watch command ps -p "$PID" -o etime