We have had theis type of question before -
Cats are pack animals, and there is a pecking order.
A maturing cat down the line will often try his luck at moving up the order by challening a senior cat. This will end in one of two ways - senior cat will inform junior that he is punching above his weight - this will take the form of a lot of noise, spitting, and yowling, the odd fight, and then things will go on as before, or -
the junior cat will manage to assert his position, which will usually be the result of a lot more yowling, spitting, and more serious fights, then some sulking and avoidance of each other, and then everything will settle down.
What you as pack leader need to do is keep out of it, let them sort it out for themselves, and everything will return to normal - until the next time.
This is all perfectly normal behaviour, and it is the way of the cat world, so resist the temptation to step in - especially in defence of the under-cat, which ever one it may be.
They must sort it out for themselves, and having done so, everyone will settle down, and life will go on.