Implant popular cereal (7)
I believe the answer is:
ingrain
'implant' is the definition.
(thesaurus)
'popular cereal' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'cereal' becomes 'grain' (synonyms).
'in'+'grain'='INGRAIN'
(Other definitions for ingrain that I've seen before include "Make a habit" , "Fix" , "Firmly instil" , "dye thus" , "Plant firmly" .)