Seed of vulgar message - not good (6)
I believe the answer is:
sesame
'seed' is the definition.
(sesame seed is a kind of seed)
'vulgar message not good' is the wordplay.
'vulgar' indicates anagramming the letters (vulgar can mean impure or inferior).
'not' suggests deleting specific letters.
'good' becomes 'g' (abbreviation).
'message' with 'g' taken out is 'messae'.
'messae' with letters rearranged gives 'SESAME'.
'of' acts as a link.
(Other definitions for sesame that I've seen before include "Open .... is a pantomime cry" , "Open ...... is magic phrase for opening door in pantomime" , "which magically opens" , "Password of the Arabian Nights" , "Opening word for the oilplant?" .)