What might make you say "don't mention it!" and "less of that!" (5)
I believe the answer is:
taboo
'don't mention it and less of that' is the definition.
'taboo' can be an answer for 'don't' (I've seen this in another clue). I am not sure about the rest of the definition.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for taboo that I've seen before include "Not permitted" , "Prohibited by religion or custom" , "Social prohibition" , "No-no" , "Prohibited by social custom" .)