Fabulous writer thus breaking a record (5)
I believe the answer is:
aesop
'fabulous writer' is the definition.
(writer of fables)
'thus breaking a record' is the wordplay.
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'breaking' indicates putting letters inside.
'record' becomes 'ep'.
'a'+'ep'='aep'
'so' put within 'aep' is 'AESOP'.
(Other definitions for aesop that I've seen before include "Great Greek teller of animal fables" , "Greek teller of animal tales with a moral" , "fabulous figure?" , "A classical fable teller" , "he wrote in Greek" .)