Work with fourth estate to subjugate (7)
I believe the answer is:
oppress
'subjugate' is the definition.
(thesaurus)
'work with fourth estate' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'with' means one lot of letters go next to another.
'fourth estate' becomes 'press' ('press' can be a synonym of 'fourth estate').
'op'+'press'='OPPRESS'
'to' is the link.
(Other definitions for oppress that I've seen before include "Keep in subjection and hardship" , "Govern with cruelty and injustice" , "Treat cruelly" , "Crush, subjugate" , "Put down" .)