It would be a great wall, however, many variables say otherwise.
Firstly, Claydol is a very vulnerable Pokemon. Despite it's high defenses it has about six different, common weaknesses. (Water, Grass, Ice, Bug, Ghost, Dark) So Hydro Pump STAB from a really well-trained sweeper (I'm not gonna say who the Pokemon will be since I know nothing about tiers xD) could quite easily) OHKO or 2HKO.
However, every Pokemon has IVs trained or not. You can use an IV calculator - it might have a very high IV stat in one - if not both - of it's defenses, which can definitely come in handy.
Also, another negative point is most Pokemon that enter a competitive battle are 99% fully trained. So, entering Claydol into a competitive battle wouldn't be the best idea.
Hope I helped. :)