• setup proxy env variables to /etc/environment file :

https_proxy=url:port
http_proxy=url:port

• execute clickable setup-lxd , then it will hang when trying to fetch apt updates
• kill process, then create as sudo a file in your container file system e.g /var/lib/lxd/containers/clickable-armhf/rootfs/etc/apt/apt.conf.d/01proxy with proxy settings:

Acquire::http::Proxy "url:port";
Acquire::https::Proxy "url:port";

• now apt update/upgrade in your container:

lxc exec clickable-armhf -- apt-get update
lxc exec clickable-armhf -- apt-get upgrade

• install click:

lxc exec clickable-armhf -- apt-get install click

it should work now.