Success — or the break formerly (8)
I believe the answer is:
hitherto
'formerly' is the definition.
(thesaurus)
'success or the break' is the wordplay.
'success' becomes 'hit' (a hit is a commercial success).
'break' indicates an anagram.
'or'+'the'='orthe'
'orthe' with letters rearranged gives 'herto'.
'hit'+'herto'='HITHERTO'
(Other definitions for hitherto that I've seen before include "Heretofore (8" , "Up to this point" , "Previously" , "Up till now" , "So far" .)