Sailor whose crime is serious (6)
I believe the answer is:
sinbad
'sailor' is the definition.
(I've seen this before)
'crime is serious' is the wordplay.
'crime' becomes 'sin'.
'is serious' becomes 'bad' (I've seen this in other clues).
'sin'+'bad'='SINBAD'
'whose' acts as a link.
(Other definitions for sinbad that I've seen before include "The 1001 Nights sailor" , "Literary sailor" , "Sailor of The thousand-and-one Nights" , "Adventurous sailor of Arabian Nights or pantomime" , "Legendary sailor from Arabian nights" .)