Certified GKH Gardening Expert
If the clover is actively growing (not going to seed), a post-emergent herbicide should take them out. If they are going to seed, the herbicide won't be as effective. Keep the lawn mowed short so as not to let the seeds mature and drop. That will help a lot next season. If the clover isn't going to seed, you can let the grass grow taller, that way shading out the horizontal clover.
The article below says selective herbicides won't kill clover and you have to use non-selective such as glyphosate, but I just looked at the label for a popular herbicide to kill weeds on lawns and it lists clover. So you can try it.