Simultaneous Multiple Rounds Auction (SMRA) and Combinatorial Clock Auction (CCA) are two most dominant types of spectrum auctions in use today. CCA has potential to surpass SMRA as the standard for spectrum auctions. CCA's first practical implementations were the Trinidad and Tobago spectrum auction in 2005 and recently, Innovation, Science and Economic Development (ISED) Canada will held an auction for 600MHz licenses using CCA. New and improved auction format and rules emerged, led by advancement in auction theories and designs. The need of well understanding about CCA is a must in spectrum auction industry. CCA consist of Allocation Stage: Clock Rounds phase and Supplementary Round phase. This paper explain the road of CCA evolution and detailed focus in complexity in clock rounds phase. Two different activity-rules in clock rounds phase has been explain in this paper. We can conclude some of the future research in Combinatorial Clock Auction: the need of tools for clock rounds phase, the component tools to solve winner determination problem in supplementary round, and the pricing rule in CCA.