Greenfield does not seem to supply workshop manuals to the public. If you contact the company and give the serial number as well as the model of your mower, they may help you with some information.
I suggest you look at this thread at Outdoorking:
https://www.outdoorking-forum.com.au/forum/u...6&Words=greenfield&Search=true#Post33655That concerns a case where a member had a problem with drive belt slip, which has much the same symptoms as clutch slip, and happens to fit your problem quite well. This tends to happen sometimes when the Greenfield automatic belt adjuster doesn't quite click forward to the next notch, and needs a bit of manual assistance. The thread includes the page from a Greenfield owner manual telling you how to provide the manual assistance required.
If the problem is not due to belt slip, you may need to reline the clutches of your mower, which is quite a difficult job, though a number of Outdoorking members have done it successfully and documented how they did it in our archives. For now, I suggest you look carefully at the belt adjustment thread. Please let us know whether the belt tensioning procedure fixes your problem. If it does not, we can talk about alternative causes.