Jack, we repeat, won't do your teeth any good (6)
I believe the answer is:
tartar
'won't do your teeth any good' is the definition.
I can't judge whether this definition defines the answer.
'jack we repeat' is the wordplay.
'jack' becomes 'tar' (both are informal terms for sailor).
'we' becomes 'ta' (I am not sure about this - if you are sure you should believe this answer much more).
'repeat' becomes 'r' (abbreviation).
'tar'+'ta'+'r'='TARTAR'
Can you help me to learn more?
(Other definitions for tartar that I've seen before include "plaque" , "She terrifies" , "Unpleasant sort" , "For instance, Genghis Khan" , "may be hard to brush off" .)