Of course its best to avoid this altogether. So having a hanky ready (that is outside of a container) and close to your hand (e.G. in your jackett/coat pocket) and apply directly after/during ejaculation will reduce the amount of situations where leftovers are on clothes. If you happen to have this kind of sex regularly this should be a permanent preparation.
To clean: First remove the semen as quickly as possible by placing your finger next to it (not on it) and roll it up while moving over the semen. Use each finger only once or clean it after use with a towel. Speed is important. Most fabric will not soak up the liquid instantly so you can reduce the intense of the spot if you remove it quicker. Semen is kinda sticky and so is your finger. This works best on jeans or similar fabric that is kinda water repelling. If available, use tampons for this task, they work even better then fingers.
The goal here is to lift the semen from the fabric, be careful not to rub it in more then absolutely necessary.
Then spit on your hand (or use water if available) and press the cleaning liquid through the fabric (from the inside) so that the ejaculate is diluted and replaced by spit / water. The goal here is to replace the highly visible white semen with a less visible kinda clear liquid.
Then wait as long as possible. Semen changes color to a semi-transparent yellow after some minutes and thus becomes less visible, also some parts dry. If possible press fabric that is not visible (socks) on the wet spot to dry it quicker. DO NOT USE PAPER TOWELS! They are likely to leave small white particles on your clothes that shine like stars on black fabric.
If nothing helped, create a distraction. If you wear a shining red flower in your jackets front pocket nobody will care about your pants. Of course that only works so long, so be sure to properly clean yourself soon or go grab your spare pants from the car.