Gaping is when a clam's incurrent siphon (the big slit) is really wide -- usually you can see the organs inside (most often, for instance, you can see a pair of white feathery looking structures just inside the siphon to either side).
There is no standard definition, though -- it is based on your observation of a particular clam and how wide it usually has its incurrent siphon. Typically in response to stress, etc.
However, some clams, and even croceas as a species, tend to gape for whatever reason.
As long as the clam fully extends its mantle and reacts to shadows I would not worry about it and just watch it for changes.
hth,
John