Stony-hearted mother and son (6)
I believe the answer is:
damson
'stony-hearted mother and son' is the definition.
I can't judge whether this definition defines the answer.
'mother and son' is the wordplay.
'mother' becomes 'dam' (female parent of an animal).
'and' says to put letters next to each other.
'dam'+'son'='DAMSON'
Can you help me to learn more?
(Other definitions for damson that I've seen before include "Type of plum" , "Sort of plum" , "Small dark-purple plum" , "Stone fruit for nomads" , "Small purple plum" .)