Sanctimonious judges -- they're dangerous in the main (7)
I believe the answer is:
pirates
'they're dangerous in the main' is the definition.
('the main' is an old name for the sea)
'sanctimonious judges' is the wordplay.
'sanctimonious' becomes 'pi' (British abbreviation for pious).
'judges' becomes 'rates' (rating is a kind of judging).
'pi'+'rates'='PIRATES'
(Other definitions for pirates that I've seen before include "More than one brigand" , "Copies" , "felons found in craft?" , "Copyright infringers" , "Plagiarists" .)