The very well known working of a sum (6)
I believe the answer is:
famous
'the very well known' is the definition.
(I've seen this in another clue)
'working of a sum' is the wordplay.
'working' indicates an anagram.
'of'+'a'+'sum'='ofasum'
'ofasum' is an anagram of 'FAMOUS'.
(Other definitions for famous that I've seen before include "Known-of widely" , "Acclaimed" , "Well known" , "Renowned, celebrated" , "Widely-known" .)