Serine protease that functions in the melanization-mediated immune response (PubMed:20953892, PubMed:25525260, PubMed:26926112). Cleaves and activates prophenoloxidase (PPO), which is required for the activation of the prophenoloxidase cascade probably following the recognition of pathogen-derived products (PubMed:20953892, PubMed:26926112). Inhibited by serpin SRPN2. Forms a covalent heterodimer with SRPN2; the interaction inhibits CLIPB9 protease activity. The clip domain consists of 35-55 residues which are 'knitted' together usually by 3 conserved disulfide bonds forming a clip-like compact structure. Proteolytic cleavage is necessary for activation. RNAi-mediated knockdown in female does not affect lifespan and basal melanization (PubMed:20953892). Simultaneous RNAi-mediated knockdown of SRPN2 and CLIPB9 in female, reduces the spontaneous melanization and the lifespan shortening occurring in SRPN2 RNAi-mediated knockdown (PubMed:20953892, PubMed:26926112). Belongs to the peptidase S1 family. CLIP subfamily.