Good-hearted mother into soft rock (5)
I believe the answer is:
magma
'soft rock' is the definition.
'magma' can be an answer for 'rock' (magma is a kind of rock). I am not sure about the 'soft' bit.
'good-hearted mother' is the wordplay.
'good hearted' becomes 'mag' (I can't justify this - if you can you should give a lot more credence to this answer).
'mother' becomes 'ma' (ma is a kind of mother).
'mag'+'ma'='MAGMA'
'into' is the link.
Can you help me to learn more?
(Other definitions for magma that I've seen before include "Molten or pasty rock" , "That's hot!" , "Glassy volcanic rock" , "crust's content" , "Lava" .)