The simple way is to use the FX Sony curves and adjust the curve to boost shadow detail while holding back highlight areas. Likely you'll see an increase in noise in the shadows. Probably you'll need Sony Levels as well. If you don't like the result because you've lost contrast in mid and high light areas (you control this with the shape of the curve) you can put a copy of the clip on a lower track and use Sony Cookie Cutter FX (or Bezier mask) on each clip to isolate the relevant areas on each track, then use the above FX to bring up the shadow areas on one clip. This will keep the well exposed section while letting you boost the underexposed in isolation. Make sure the clips are perfectly synchronised and blur the edges of the mask. Trial and error. If the camera is moving, keyframes.