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Secondary sources are sources you have read about through the works of others. If you read a work by Kagan (1984) in which Ainsworth (1982) was cited, and you were unable to to read Ainsworth’s work yourself, cite  Ainsworth’s work as the original source followed by Kagan’s work as the secondary source. Only Kagan’s work appears in the reference list (not Ainsworth’s) since you found the information in Kagan. Use the following form:

(Ainsworth, 1982, as cited in Kagan, 1984) demonstrated…

If the year of the primary source is unknown, omit it from the in-text citation.

Allport’s diary (as cited in Nicholson, 2003)

(American Psychological Association,  2020, p. 258)