It is possible that the ace hit him, but it is also possible that he is using it as a scare card or maybe also caught 4 clubs. Without anything much showing out there, I would look to call there more than fold. It just seems a little weakish to fold. Granted, the pair of tens is certainly marginal here at this point, but a lot of times you just have to play with these marginal situations until you see something to tell you otherwsie.