Firmly establish new cereal crop on island (7)
I believe the answer is:
ingrain
'firmly establish' is the definition.
(I've seen this in another clue)
'new cereal crop on island' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'cereal crop' becomes 'grain' (I've seen this before).
'on' means one lot of letters go next to another.
'island' becomes 'i' (geographical abbreviation).
'n'+'grain'='ngrain'
'ngrain' after 'i' is 'INGRAIN'.
(Other definitions for ingrain that I've seen before include "Fix" , "deeply impressed" , "Make a habit" , "Plant firmly" , "Raining? (anagram)" .)