G;day megana.
Firstly let me Welcome you to the forum.
Clutch.
When you buy a new Massey clutch it is held compressed by three 1/4 unc screws to allow assembly to the flywheel it is possible the screws have not been removed after clutch installation, to check remove the plate on the bottom of the clutch housing ,the one held on by four bolts, make sure you remove the ignition key BEFORE you go putting your hands in there.
With a Flashlight ,a big screwdriver a 7/16 " ring spanner, a Dumpy screwdriver and a wee bit of patience you will be able to remove the screws they will have either a hex head or a screwdriver slot..
The other thing that would cause the clutch to slip although it would not make the pedal light is it could possibly be lack of running CLEARANCE ,to check ,stand on the left hand side of the tractor and with your right hand apply light pressure to the clutch pedal ,when lightly pressed down there should be a gap between the footpeg and the pedal of between 3/16" and 3/8" if there is no clearance loosen the pinch bolt that locks the pedal to the shaft and gently push it down till you get the correct clearance,lock the bolt and try it.
Regards .
Hutch.