Mum is collecting good, 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'm not sure about the 'soft' bit.
'mum is collecting good' is the wordplay.
'mum' becomes 'mama' (synonyms).
'is collecting' is an insertion indicator.
'good' becomes 'g' (abbreviation).
'mama' enclosing 'g' is 'MAGMA'.
Can you help me to learn more?
(Other definitions for magma that I've seen before include "Lava" , "Semi-fluid material within the earth's crust" , "Hot fluid rock" , "That's hot!" , "Glassy volcanic rock" .)