Before revision:  That is partitioning a graph into exactly p or a minimal resp. maximal number of clusters such that every cluster fulfills the following weight condition: the weight of each cluster should to be at least l and at most u. These kind of partitioning problems are NP-hard for series-parallel graphs but can be solved in polynomial time on trees.
After revision:  That is partitioning a graph into exactly p or a minimal resp. maximal number of clusters is NP-hard for series-parallel graphs but can be solved in polynomial time on trees.