Number eighteen is well known (7)
I believe the answer is:
notable
'well known' is the definition.
(I've seen this before)
'number eighteen' is the wordplay.
'number' becomes 'no' (abbreviation for 'number').
'eighteen' becomes 'table' (I can't explain this - if you can you should give a lot more credence to this answer).
'no'+'table'='NOTABLE'
'is' is the link.
Can you help me to learn more?
(Other definitions for notable that I've seen before include "well known" , "Unusual" , "VIP" , "Celebrated" , "Famous or memorable" .)