Mobile ad hoc networks are dynamically organized by a collection of wireless mobile nodes. The mobile nodes in ad hoc networks can move arbitrarily thus the topology of network changes dynamically. Due to the properties of communication medium in wireless networks, unidirectional links may exist between mobile nodes thus results in the difficulty of link utilization and routing. In this paper, we take the advantages of multi-hop acknowledgement and employ the clustering technique to design an efficient hybrid routing protocol in ad hoc networks with unidirectional links. Simulation results demonstrate the stability and efficiency of the proposed protocol.