However classy my side’s boldness, say nothing! (6,4,3)
I believe the answer is:
button your lip
'say nothing' is the definition.
'however classy my side's boldness' is the wordplay.
I cannot really understand how this works, but
'however' could be 'but' (I've seen this before) and 'but' is found within the answer.
'classy' could be 'u' (I've seen this in other clues) and 'u' is present in the answer.
'boldness' could be 'lip' and 'lip' is found within the answer.
'side' could be 'r' (abbreviation for right) and 'r' is located in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for button your lip that I've seen before include "Shut up" , "Wrap up" .)