A light string going over a clamped pulley of mass m supports a block of mass M as shown in figure. The force on the pulley by the clamp is given by

 

$\mathrm{T} =\mathrm{mg}$

Force on the pulley ( other than from clamp)

${\mathrm{F}}_{\mathrm{net}}=\sqrt{{\left(\mathrm{T}+\mathrm{mg}\right)}^2+{\mathrm{T}}^2}=\mathrm{g}\sqrt{{\left(\mathrm{M}+\mathrm{m}\right)}^2+{\mathrm{M}}^2}$

Since, pulley is in equilibrium, clamp will exert the same amount of force in opposite direction. Or pulley will also exert this much force on clamp.