Title thus originally adopted by Scrooge (6)
I believe the answer is:
mister
'title' is the definition.
'thus originally adopted by scrooge' is the wordplay.
'originally' says to take the initial letters.
'adopted by' indicates putting letters inside (I've seen this in other clues).
'scrooge' becomes 'miser' (synonyms).
The first letter of 'thus' is 't'.
't' going inside 'miser' is 'MISTER'.
(Other definitions for mister that I've seen before include "that's what you'd call him" , "Bloke" , "Mr" , "Male form of address" , "Form of address to a man, used without his name" .)