Greek character with naughty crime at sea (6)
I believe the answer is:
piracy
'crime at sea' is the definition.
'greek character with naughty' is the wordplay.
'greek character' becomes 'pi' (letter from the Greek alphabet).
'with' means one lot of letters go next to another.
'naughty' becomes 'racy' (synonyms).
'pi'+'racy'='PIRACY'
(Other definitions for piracy that I've seen before include "Robbery on the high seas" , "Buccaneering" , "theft" , "Illegal copying" , "Robbing of ships at sea" .)