Something growing in the wood (5)
I believe the answer is:
'wood' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'something growing in' is the wordplay.
I cannot really understand how this works, but
'in' is within the answer.
This may be the basis of clue (or it may be nonsense).
'the' is the link.
Can you help me to learn more?
(Other definitions for grain that I've seen before include "Structure of wood" , "Cereal crops" , "Small particle - weight" , "Tiny amount" , "Character" .)