Best phone company accepts ninety pounds (5)
I believe the answer is:
excel
'best' is the definition.
(thesaurus)
'phone company accepts ninety pounds' is the wordplay.
'phone company' becomes 'EE' (British mobile network EE).
'accepts' indicates putting letters inside.
'ninety' becomes 'XC' (Roman numerals).
'pounds' becomes 'L' (L can represent the monetary pound).
'ee' enclosing 'xc' is 'exce'.
'exce'+'l'='EXCEL'
(Other definitions for excel that I've seen before include "Be exceptionally good (at something)" , "Eclipse" , "Do brilliantly" , "Be very good at, better than others" , "Perform exceptionally well" .)