How do I temporarily bypass an alias in tcsh?

You can use a backslash:

% alias ls ls -a
% ls
# ls -a output here
% \ls
# plain ls output here

For shell builtins, there turns out to be a gotcha: a leading backslash prevents both aliases and builtins from being used, but an internal backslash suppresses aliasing only.

% alias cd pushd
% cd /tmp
/tmp /tmp 
% c\d
% dirs
~ /tmp

(I'm tempted to call that another argument against using the csh family of shells.)