Discourage work on fourth estate (7)
I believe the answer is:
oppress
'discourage' is the definition.
(thesaurus)
'work on fourth estate' is the wordplay.
'work' becomes 'op' (common abbreviation for 'opus').
'on' says to put letters next to each other.
'fourth estate' becomes 'press' (synonyms).
'op'+'press'='OPPRESS'
(Other definitions for oppress that I've seen before include "Govern harshly, weigh down with care" , "Treat tyrannically" , "Subjugate by force" , "Cause to suffer" , "'Persecute, come down heavy on (7)'" .)