Swindler almost entirely ruthless, sadly (7)
I believe the answer is:
hustler
'swindler' is the definition.
(both can mean a fraud)
'almost entirely ruthless sadly' is the wordplay.
'almost entirely' means to remove the last letter.
'sadly' is an anagram indicator.
'ruthless' with its last letter removed is 'ruthles'.
'ruthles' anagrammed gives 'HUSTLER'.
(Other definitions for hustler that I've seen before include "Prostitute" , "Conman" , "Aggressive seller" , "Swindler" , "Illicit dealer (mainly US)" .)