would a renault megane 225 turbo engine fit into a clio cup 172

No, l do not think it fits and l defintely DO NOT recommend such modification.

you can make anything fit anything.ide give it a go.

In theory. yes. They are both Renault is F4R engine which is based on the same engine block. The clio is an F4R and the megane is an F4Rt.
But l do not think it will be an easy engine change. But why settle for
225 bhp, when you can have 300 bhp with a k-tec racing turbo conversion !

l know people who have looked into this, and they say its just to tight. converting it to a VAG 1.8t unit is quite popular.

i dont know why you want to do it though, the 172 cup is great as it is. its not all about raw power, its about how you put it down onto the road, and the 172 does this better than a lot of cars in higher classes.