It’s well known: new lockdown meme no good (6,9)
I believe the answer is:
common knowledge
'it's well known' is the definition.
'new lockdown meme no good' is the wordplay.
'new' indicates an anagram.
'good' becomes 'g' (abbreviation).
'lockdown'+'meme'+'no'+'g'='lockdownmemenog'
'lockdownmemenog' anagrammed gives 'COMMON KNOWLEDGE'.
(Other definitions for common knowledge that I've seen before include "understood by all" , "generally accepted" .)