Bit loopy, endlessly untrue rambling - best say no more! (6,4,3)
I believe the answer is:
button your lip
'best say no more' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'bit loopy endlessly untrue rambling' is the wordplay.
'endlessly' means to remove the last letter.
'rambling' is an anagram indicator.
'untrue' with its final letter taken away is 'untru'.
'bit'+'loopy'+'untru'='bitloopyuntru'
'bitloopyuntru' is an anagram of 'BUTTON YOUR LIP'.
Can you help me to learn more?
(Other definitions for button your lip that I've seen before include "Shut up" , "Wrap up" .)