Well-known but lacking in brains (7)
I believe the answer is:
notable
'well-known' is the definition.
(thesaurus)
'lacking in brains' is the wordplay.
'lacking' becomes 'no' (eg lacking money means with no money).
'in brains' becomes 'table' (I can't explain this - if you can you should give a lot more credence to this answer).
'no'+'table'='NOTABLE'
'but' is the link.
Can you help me to learn more?
(Other definitions for notable that I've seen before include "Significant or distinguished" , "celebrity" , "Worthy of attention, significant" , "Celebrated" , "Star" .)