Nope, no Clear Stack.
Reason for this is that there was no such function on the HP32SII and the HP33S is an almost exact duplicate of the HP32SII, with only a few things added.
So, if it wasn't on the HP32SII, it probably isn't on the HP33S. :-)
Don't forget the learning modules on HP's site for the HP33S that are also a source of a lot of information:
http://www.hp.com/calculators/scientific/33s/technical.html
Gene
There is no key for this function, but it is trivial to program it.
My 33S is programmed for "Clear Stack" by hitting XEQ N:
LBL N
0
ENTER
ENTER
ENTER
RTN
It is also programmed for "Clear Stack, All Variables, and Statistics Registers" by hitting XEQ P:
LBL P
XEQ N
CLVARS
CL-Sigma
RTN
The only problem is that such programs use up the scarce labels on the 33S. If you plan to store a lot of other programs in your 33S, then you may not want to waste labels on simple programs like these.