In sum, oaf could become notorious (8)
I believe the answer is:
infamous
'notorious' is the definition.
('infamous' can be a synonym of 'notorious')
'in sum oaf could become' is the wordplay.
'could become' indicates anagramming the letters.
'sum'+'oaf'='sumoaf'
'sumoaf' is an anagram of 'famous'.
'in'+'famous'='INFAMOUS'
(Other definitions for infamous that I've seen before include "Notorious, having very bad reputation" , "Of evil repute" , "Notorious, having evil reputation" , "Dishonourable" , "Heinous" .)