As far as I know, you can abandon the quest yes, but that quest will be forfeit for the clan (i.e. no-one else can take it).  

sure but how to do it ? i cant see this option anywhere

I do not think that option is there yet.

The only way i have seen quest abandoned is when people leave the Clan.

Just wait till next weeks reset and then you can pick up new CQ

the new heights in underdeveloped features have been achieved with this clan quests xd.

Hi @Kifon! As of right now, there is no direct possibility to resign a Clan Quest once you've taken it. That's why we ask to carefully consider your ability to complete it before pursuing it.

I understand that it might feel a little bit weird that a player cannot resign from a Quest. But the current mechanics encourages commitment to the Quest once a player started it

Have a nice day! 

hi, if u leave the clan and joined another . still can i complete quests i take?

No, you cannot