If he goes outside this could be from a cat fight, or something he squeezed into/through that scraped him. Or it could be any number of other things. Is it sensitive to the touch? It doesn't seem like there is any blood, unless you wiped that area off. For now, if you have access to chamomile tea, you can try that to see if it helps clean it up.
Buy chamomile tea bags - you can find them in most any grocery store and generally speaking those are Germain - you don't want English or any flavorings or additives. Brew a bag in water, cool the liquid to room temp and then dab some on that spot with cotton balls or a soft cloth - 2-3 times a day.
Chamomile tea, which is a soothing and healing agent, also has anti-bacterial/anti-fungal properties. It is safe for your cat to ingest should he do so.
In the meantime, don't let him back outside if you think that is where he got injured or got into something.
Yeah he’s an outdoor cat with a habit of fighting and hunting, there was no blood just clear stuff, I’ve locked his cat flap and he’s in the for foreseeable, I will buy chamomile tea in the morning. Thank you for your help
This depends on what bit him, if an insect, or what attacked him, if a larger animal. Watch him really carefully for signs of infection, swelling, general discomfort. If this is from a cat bite, the bacteria easily enters the skin as cats' teeth are like needles and the skin closes very quickly over them; not seeing blood is not always the determining factor.