Mike, if you follow that process and the belt is cleared of possible blame, the clutch will be the only potential cause left. However the clutch can only slip if you have run out of pedal movement in the forward direction: until that point is reached, you just push the pedal further until it stops slipping. Also, that clutch is designed to slip continuously for long periods without harm (except for gradual wear of the clutch lining). So, if the problem is clutch slip, you should be conscious of not being able to push the pedal any further, while it is happening.
I don't know how long you've had the mower and how much you know about its past maintenance. We have had one case where it looks as if someone ran a Greenfield long after the forward clutch was worn out - presumably making the shim stack thicker and thicker as he ran out of pedal movement each time - until the lining was gone completely and the metal clutch disk made contact with the pulley surface, damaging it fairly severely. See Millsi's thread here:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=43216&Searchpage=1&Main=7862&Words=%2Bgreenfield+%2Blathe+%2Bclutch&Search=true#Post43216
Whether the problem is with the belt or the clutch, it is important to fix it, or there may be damage to the pulley (if it is belt slip) or the clutch (if it runs out of clutch lining material). However it seems to me your best clue as to whether you have a clutch fault, will be whether the pedal is fully floored when the squeal happens. A clutch squeal with a partly-pressed pedal just means you have a slightly glazed clutch and need to push the pedal a bit further to wear off the glazing. I think glazing with a Greenfield is caused by running the engine faster than necessary, so the percentage of clutch slip when you are mowing is higher than necessary. This causes clutch lining wear as well as glazing. It is best to set the engine speed as low as possible whilst having enough power to do the job, so that slip can be minimised.
