Abuse: polite not to speak if you have it, they say (8)
I believe the answer is:
mouthful
'abuse polite not to speak if you have it' is the definition.
I can't judge whether this definition defines the answer.
'they say' is the wordplay.
I cannot really understand how this works, but
'say' could be 'mouth' (I've seen this in other clues) and 'mouth' is present in the answer.
The remaining letters 'ful' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for mouthful that I've seen before include "Outburst of strong language" , "Difficult word or phrase to pronounce" , "Complicated word or phrase" , "Something difficult to say or understand" .)