That depends on which soecies you want to keep...... basically there are 2 very common less light demanding clams species(derasa and squamosa) and 2 ratehr common high light deamnding clam species(crocea and maxima)...... I left out gigas as it is not very common here and do not have appealing colour which most reefer go after.
Even for derasa and squamosa(low light demanding clam), the least light kind you can keep them under is t5s and usually they does better in MH....... I have seen ppl having success keeping clams under PL by placing them high up but thats not really advisable.
For high light demanding clam, The least light kind you can go for is no other than t5s and even so you have to squeeze many t5s in one tank just to keep the clam alive thus it is best recommended to keep them under MH........ Read up somemore, ultra maxima require at least >250watt of MH to thrive properly but nevertheless if you place the clam high up, 150watt should do.....