When enabling freshclam.service the following warning is displayed:

The unit files have no installation config (WantedBy=, RequiredBy=, UpheldBy=,
Also=, or Alias= settings in the [Install] section, and DefaultInstance= for
template units). This means they are not meant to be enabled or disabled using
systemctl.

Possible reasons for having this kind of units are:
* A unit may be statically enabled by being symlinked from another unit's
  .wants/ or .requires/ directory.
* A unit's purpose may be to act as a helper for some other unit which has
  a requirement dependency on it.
* A unit may be started when needed via activation (socket, path, timer,
  D-Bus, udev, scripted systemctl call, ...).
* In case of template units, the unit is meant to be enabled with some
  instance name specified.

When looking is /usr/lib/systemd/system/freshclam.service the following is
missing (copied from :/usr/lib/systemd/system/clamd.service)

[Install]
WantedBy=multi-user.target