Firmly establish in cultivated cereal (7)

I believe the answer is:

ingrain

I'm a little stuck... Click here to teach me more about this clue!

'firmly establish' is the definition.
(I've seen this in another clue)

'in cultivated cereal' is the wordplay.
'cultivated cereal' becomes 'grain' (synonyms. I am not sure about the 'cultivated' bit.).
'in'+'grain'='INGRAIN'

Can you help me to learn more?

(Other definitions for ingrain that I've seen before include "Firmly instil" , "Drive home" , "Plant firmly" , "Impress deeply on the mind" , "Firmly establish (habit)" .)

I've seen this clue in the Sydney Morning Herald.

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