I will add the comment you just wrote above. Since you are not sure that this is clear I could add something even more verbose.

"Cluster i can be thought to have i_weight points each having a similarity with cluster x equal to sim[i,x]. Cluster j can be thought to have j_weight points each having a similarity with cluster x equal to sim[j,x]. Merging clusters i and j the new cluster will have i_weight points with similarity sim[i,x] and j_weight points with similarity sim[j,x] the average similarity of the new cluster with cluster x is (sim[i,x]*i_weight + sim[j,x]*j_weight) / (weight_sum)"