As Amherst pointed out in weekly rewards suggestion post - there is no way for a clan to make gold, except by getting clan mates to pay taxes.

My Recommendation:
Weekly Clan Challenges.
For example:
Get 20 Island Pro Quests a week - Get 5G
Get 20 Island Hero Quests a week - Get 5G
Get 20 Ropes - Get 5G
Get 20 Daymen - Get 5G.

etc, etc. There are many weekly quests/challenges that could be used to get a little reward to clans for being active.