Request the days leave as usual and then while the leave request is still in Draft click on the 'Change' link next to the Duration of the leave.  (You can also come down the page a little bit to 'Leave Request Days and click the 'Change Leave Days' button).

On the new page that opens you will see your working day is split onto 2 periods, morning and afternoon. 

So:

If you want to take leave in the afternoon only just change the morning period (i.e. period 1 start and end) to 00:00.  REMEMBER: you want to leave the time in for the period you want to take leave for. 

If you want to book an hour in the morning change the afternoon period (i.e. period 2 start and end) to 00:00 and change the period 1 end to 10:00.

I have assumed a fairly common work pattern above of 09:00 - 17:30 with a lunch break between 13:00 - 14:00 but the same principle applies to all part day requests.