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Okay, so here one possible solution:

// given your mousepoint coordiantes x,y (e.g. through setMouseCallback)
cv::Point mouse(x,y);
int nearest_index = -1;
float min_dist = std::numeric_limits<float>::max();
for( size_t i = 0; i < keypoints.size(); i++ ) {
  double distance = cv::norm(mouse - keypoints[i].pt);
  if ( distance < min_dist ) {
    min_dist = distance;
    nearest_index = i;
  }  
}
cv::KeyPoint nearest = keypoints[nearest_index];

As also said in the comments, you can also use BFMatcher or FlannMatcher if this solution is too slow for you.