Well, if you want to gain a lot of clintz you must buy credits from the shop...
If you don't want or can't do so, another way is by playing in DT (remember the fact that you receive 50clintz just for participating)
Also, playing in survivor is a nice option
If you can sell your card, trying ot buy cards from market when new cards come out and then sell that card one week after that is a good idea too
you can get 6K pretty fast, but as long as you have money to buy those cheap cards