Painful experience, heartless sort of 3 (4)
I believe the answer is:
oral
'sort of 3' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?
'painful experience heartless' is the wordplay.
'painful experience' becomes 'ordeal' (I've seen this before).
'heartless' means to remove the middle letters.
'ordeal' with its centre removed is 'ORAL'.
Can you help me to learn more?
(Other definitions for oral that I've seen before include "Type of examination" , "By word of mouth (4)" , "Vocal test" , "See 18" , "Using speech rather than writing" .)