An in situ method for forming a thermally conductive thermal radical cure silicone composition is provided. The in situ method comprises forming a thermally conductive clustered functional polymer comprising the reaction product of a reaction of a polyorganosiloxane having an average, per molecule, of at least 2 aliphatically unsaturated organic groups; a polyorganohydrogensiloxane having an average of 4 to 15 silicon atoms per molecule; and a reactive species having, per molecule, at least 1 aliphatically unsaturated organic group and 1 or more curable groups; in the presence of a filler treating agent, a filler comprising a thermally conductive filler, an isomer reducing agent, and a hydrosilylation catalyst. The method further comprises blending the thermally conductive clustered functional polymer with a radical initiator.