Does this card allow you to pay a cost for fences with combined wood and clay resources? Ie: I want 6 fences, I pay 4 wood and 2 clay.
Absolutely, yes. The wording of the card is explicit: You can use clay instead of wood to build fences. There are no further caveats or limitations. It doesn't say only clay, nor does it in any way indicate that you can no longer use wood, nor does it specifically say you can't use both simultaneously.
Truthfully, the card would be relatively useless otherwise.