Endonuclease which cleaves flap structures at the junction between single-stranded DNA and double-stranded DNA. Specific for 5-overhanging flap structures in which the 5-upstream of the flap is completely double-stranded. Prefers the blocked-flap structures similar to those occurring at replication forks, in which the 5 single-strand overhang of the flap is double-stranded (By similarity).