To go beyond forty in Roman numerals, we hear (5)
I believe the answer is:
excel
'to go beyond' is the definition.
(thesaurus)
'forty in roman numerals we hear' is the wordplay.
'forty in roman numerals' becomes 'XL'.
'we hear' shows a homophone (sound like).
'xl' sounds like 'EXCEL'.
(Other definitions for excel that I've seen before include "Be very good at, better than others" , "Do really well as at exams" , "Surpass; shine" , "Do exceedingly well" , "Every one" .)