How is TMP cleared

That depends on your distribution. On some systems, it's deleted only when booted; others have cron jobs deleting items older than n hours.

• On Debian and derivatives
  • On Debian and in general, on boot rules are defined in
    • /etc/default/rcS.
• On Ubuntu since version 15.10 and its derivatives: using tmpfiles.d.
  • The default tmp.conf only clears /tmp on boot.
  • For more details, see this answer.
• On Ubuntu 15.04 and before: - using tmpreaper 2
  • which gets called by /etc/cron.daily,
  • configured via /etc/default/rcS and /etc/tmpreaper.conf. (Credits to this answer).
• On distributions from Red Hat and its derivatives: by age
  • RHEL 7, 8 and others with SYSTEMD
    • configured via /usr/lib/tmpfiles.d/tmp.conf,
    • called by systemd-tmpfiles-clean.service.
  • RHEL 6
    • used /etc/cron.daily/tmpwatch.
• On Gentoo:
  • used /etc/conf.d/bootmisc.

Example stock /usr/lib/tmpfiles.d/tmp.conf

#  This file is part of systemd.
#
#  systemd is free software; you can redistribute it and/or modify it
#  under the terms of the GNU Lesser General Public License as published by
#  the Free Software Foundation; either version 2.1 of the License, or
#  (at your option) any later version.

# See tmpfiles.d(5) for details

# Clear tmp directories separately, to make them easier to override
q /tmp 1777 root root 10d
q /var/tmp 1777 root root 30d

# Exclude namespace mountpoints created with PrivateTmp=yes
x /tmp/systemd-private-%b-*
X /tmp/systemd-private-%b-*/tmp
x /var/tmp/systemd-private-%b-*
X /var/tmp/systemd-private-%b-*/tmp

# Remove top-level private temporary directories on each boot
R! /tmp/systemd-private-*
R! /var/tmp/systemd-private-*