You can use just the single value from a dataset with the range selector option for datasets. Here's a simple example:

3dcalc -a clusts+tlrc'<1>' -expr a -prefix 'clust1'

The range selectors use angle brackets (less than and greater than) to specify a lower and upper limit ("<minvalue..maxvalue>").

You can also use the equals operator in 3dcalc's expression 'equals(a,1)'