Press onwards to tackle United on park (6,6)
I believe the answer is:
fourth estate
'press' is the definition.
(synonyms)
'united on park' is the wordplay.
'united on' becomes 'fourth' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'park' becomes 'estate' (I've seen this before).
'fourth'+'estate'='FOURTH ESTATE'
'onwards to tackle' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for fourth estate that I've seen before include "Journalistic profession - press" , "addition to three-car family?" .)