Sanctimonious seaman stealing (8)
I believe the answer is:
pirating
'stealing' is the definition.
(pirating is a kind of stealing)
'sanctimonious seaman' is the wordplay.
'sanctimonious' becomes 'pi' (British abbreviation for pious).
'seaman' becomes 'rating' (British term for a naval enlisted man).
'pi'+'rating'='PIRATING'
(Other definitions for pirating that I've seen before include "running off illicitly" , "bootlegging" .)