1995-2002 Range Rover P38A
Mr/ Ms Guest is almost right!If it is too large, hold the clutch outer and remove the central nut. The clutch outer is on splines and simply pulls off and there are shims behind it. Remove shims until the gap is correct.
It's the inner that you need to hold (the bit that the belt doesn't run round and spins when you turn it with your fingers), three nice handy holes for a pin spanner or alternative in it for that purpose. Then remove it, then adjust shim to suit.