Something like the following should work, you'll have to make sure the table names are aligned the way you'd like.
proc sql;
create table tmpcluster2 as
select min(a.to_id) as to_id, a.from_id
from tmpcluster a
where threshold_ind=1
group by a.from_id;
create table tmpcluster3 as
select to_id, min(from_id) from_id
from tmpcluster2
group by to_id;
quit;