Ground corn's right stuffed into marrow (5)
I believe the answer is:
grist
'ground corn's' is the definition.
'grist' can be an answer for 'corn's' (I've seen this before). I am not sure about the 'ground' bit.
'right stuffed into marrow' is the wordplay.
'right' becomes 'r' (common abbreviation).
'stuffed into' means one lot of letters goes inside another.
'marrow' becomes 'gist' (synonyms).
'r' put inside 'gist' is 'GRIST'.
Can you help me to learn more?
(Other definitions for grist that I've seen before include "Grain for grinding" , "seed corn" , "may be put through the mill" , "Corn [to the mill]" , "what miller needs" .)