DISPLAY=:0 aticonfig --od-enable --adapter=all
DISPLAY=:0 aticonfig --od-setclocks=1000,300 --adapter=x
1000=Core clock
300=Mem clock
x=adapter number
Type that in Terminal, should work.
Don't use AMDOverclockCtrl at the same time, it messes up the clocks sometimes.
edit: Oh, you are using 11.4, well probably won't work then. Use 11.6 instead or wait for 11.7.