I think we need more info on this Rosie. Clone tool is "S" on the keyboard (Stamp tool). Make sure You are on the same layer and that you are working within a selection area if you have a selection activated and it's not hidden from sight "H". You need to first select the area to use as a clone by holding Alt key and LMB (Left Mouse Button) clicking and then pass over the area you want cloned. Make sure opacity and flow are set correctly in the top menu for the job. Yes, you can clone between layers but this is not what you are asking...as far as I know.