The person let into the secret - an islander (6)
I believe the answer is:
cretan
'the person let' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'into the secret an islander' is the wordplay.
'into' indicates the answer is hidden within the clue.
'CRETAN' is hidden in the letters of 'secret an islander'.
Can you help me to learn more?
(Other definitions for cretan that I've seen before include "A Mediterranean islander" , "One comes from a Mediterranean island" , "Native of Greek island south of mainland (6)" , "Of a Greek island, centre of the Minoan civilisation" , "Greek islander" .)