To be heartless, old and dirty is an unusual thing (6)
I believe the answer is:
oddity
'unusual thing' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for oddity that I've seen before include "Unusual thing" , "rare thing" , "Something strange or eccentric" , "Curious item" , "Quality of strangeness" .)