Old forum was surprisingly well-known (5-6)
I believe the answer is:
world-famous
'well-known' is the definition.
Both the answer and definition are adjectives.
Perhaps you can see a link between them that I don't see?
'old forum was surprisingly' is the wordplay.
'surprisingly' indicates anagramming the letters (a surprising ordering of the letters).
'old'+'forum'+'was'='oldforumwas'
'oldforumwas' is an anagram of 'WORLD-FAMOUS'.
Can you help me to learn more?
(Other definitions for world-famous that I've seen before include "Very well known" , "Known globally" .)