Clean and sand, as best you can, the backside of the clapboard.
Insert a slat of wood (1/2" ply 6" long) behind the hole, with glue on
each the slat & the clapboard surface, and hold it in place by
screwing another slat, laid on the outside face, to the inside slat.
The screw, when tightened, will pull the inside slat tight against the
backside of the clapboard. Once the inside glued slat has dried to
the clapboard, you have a solid back foundation to help install any
filler or plug type repair. *Insert the screw into the outside slat,
in prep for the attachment to the inside slat.