Identify the complete adjective clause.
he is the one for whom the message was intended.
he is the one
he is the one for whom
the one for whom
for whom the message was intended
for whom the message
the one for whom the message was intended
whom the message
message was intended

An adjective clause is the clause that has the same function as an adjective; that is it is the clause that could be substituted by the word "good".(good is an adjective).

in the following sentence:
He is the one for whom the message was intended.

 the adjective phrase is  for whom the message was intended.
It could be replaced with "good": he is the good one.