The internal flow structure and loss generation mechanism of a single-stage compressor during windmilling were investigated through experimental and computational analyses. The windmilling state occurs when the air flowing through an unlit engine drives the compressor rotor blades, similar to a turbine. This study focused on the effect of tip clearance size on the internal flow during windmilling operations. Therefore, detached eddy simulations were conducted at three tip clearance conditions: without clearance, design clearance, and wide clearance. Consequently, the total pressure loss decreased when the size of the tip clearance was expanded under windmilling conditions. In the windmilling state, separation on the pressure side due to the high negative incidence was generated, which was the main reason for the total pressure loss. When the size of the clearance increased, the size of the separation decreased because of the tip leakage flow. According to the detailed numerical results for the windmilling state, the leakage flow held the separation to the blade surface near the tip area. In addition, the tip leakage flow formed a blockage in the midchord, so the tip side of the separation vortex was moved to the midchord and formed a significant loss region. Thus, the tip leakage flow reduced the loss region, and the overall loss was reduced.