It's a crime, essentially, to defame person from Achill (8)
I believe the answer is:
islander
'person from achill' is the definition.
I can't judge whether this definition defines the answer.
'it's a crime essentially to defame' is the wordplay.
'it's a crime' becomes 'sin'.
'essentially' means to look at the middle letters.
'to defame' becomes 'slander' (synonyms).
The middle letter of 'sin' is 'i'.
'i'+'slander'='ISLANDER'
Can you help me to learn more?
(Other definitions for islander that I've seen before include "Inhabitant of Skye, say" , "one from Jersey, perhaps" , "Orcadian perhaps" , "Samoan, maybe" , "Perhaps Sardinian" .)