Very important reading matter for children engrosses son (6)
I believe the answer is:
cosmic
'very' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?
'important reading matter for children engrosses son' is the wordplay.
I cannot really understand how this works, but
'matter' could be 'sm' (Sm is an example) and 'sm' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for cosmic that I've seen before include "Johann Sebastian --" , "To do with the whole universe" , "Great" , "Of the entire universe" , "Enormous" .)