In this paper, we obtain a bound on the error term of the solution to a control constrained, linear-quadratic optimal control problem over a clustered consensus network. For large scale systems, the solution may be regarded as computationally infeasible due to the increase in dimensionality. However, the two time-scale property of the network that arises from the cluster formation indicates that the optimal control problem can be written in standard singularly perturbed form. Thus, we are able to obtain a reduced dimension optimal control problem over a reduced dimension network where individual nodes within a cluster are collapsed into an aggregate node. The solution to the reduced problem can be shown to be asymptotically equivalent in the singular perturbation parameter δ to the solution of the original problem. However, for many values of δ this asymptotic result may fail to be of practical use. We improve on this result by applying a duality theory to the clustered network and derive an upper bound χu(δ) and lower bound χ l (δ) on the solution to the optimal control problem that holds for arbitrary δ and, furthermore, satisfies the inequality |χu(δ) − χ l (δ)| = O(δ) as δ → 0.