An indexable cutting insert has mutually perpendicular longitudinal, lateral and vertical axes. The insert includes two opposite cutting portions and a clamping portion therebetween, arranged parallel to the longitudinal axis. Each cutting portion includes a rake surface and front and side relief flanks, and a cutting edge formed at an intersection of the rake surface and front relief flank. A second width of the clamping portion is greater than the first width of the cutting edges. The clamping portion has two resilient beams and a flexibility aperture opening out to side abutment surfaces of the clamping portion, allowing elastic deformation of the insert along the vertical axis. The cutting insert may further include two opposite stopper portions, each formed on a respective insert end surfaces, each stopper portion forming a step in the respective insert end surface. A cutting tool clamps one or more inserts at their clamping portions.