Sensors in wireless sensor network (WSN) usually spatially spread across geographical locations. They may be placed randomly or in an initially organized manner to cooperatively monitor certain physical or environmental phenomena. They have limited transmission powers due to their small sizes and battery constraints. Some sensors may not be able to send data directly to the sink for processing and analysis. This has led to, for example, the design of tree-based structure for delivering data over multiple hops to reach the sink. In this paper, a novel lifetime extending heuristic (MLC-X) is proposed for tree-based multi-level clustered wireless sensor network. Duties of nodes at bottlenecks in tree are modified for sustaining longer network lifetime. And the simulation results indicate that the heuristic can successfully extend life spans of sensor networks.