Promising characteristic of heartless lady (6)
I believe the answer is:
likely
'promising' is the definition.
(similar in meaning)
'characteristic of heartless lady' is the wordplay.
'characteristic of' becomes 'like' (being like something is being characteristic of it).
'heartless' says to hollow out the word (remove centre letters).
'lady' with its centre taken out is 'ly'.
'like'+'ly'='LIKELY'
(Other definitions for likely that I've seen before include "Has a good chance of being the case" , "Probable, expected" , "In prospect" , "Quite possible" , "Promising, probably" .)