Firmly established writer goes after Irish butter factory (9)
I believe the answer is:
implanted
'firmly established' is the definition.
(thesaurus)
'writer goes after irish butter factory' is the wordplay.
'writer' becomes 'ed' (I've seen this in other clues).
'goes after' means one lot of letters go next to another.
'irish butter' becomes 'im' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'factory' becomes 'plant' (a manufacturing plant).
'im'+'plant'='implant'
'ed' after 'implant' is 'IMPLANTED'.
Can you help me to learn more?
(Another definition for implanted that I've seen is " Embedded".)