To tease a lord is obscene (6)
I believe the answer is:
ribald
'obscene' is the definition.
(I've seen this before)
'to tease a lord' is the wordplay.
'to tease' becomes 'rib' (I've seen this before).
'lord' becomes 'ld' (abbreviation for the aristocratic title).
'rib'+'a'+'ld'='RIBALD'
'is' acts as a link.
(Other definitions for ribald that I've seen before include "near the knuckle" , "Bawdy and vulgar" , "Obscene" , "Coarsely mocking" , "Foul-mouthed" .)