Terrible argument, running along Hyde Park (6,3)
I believe the answer is:
rotten row
'running along hyde park' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'terrible argument' is the wordplay.
'terrible' becomes 'rotten' (I've seen this in another clue).
'argument' becomes 'row' (to row is to argue).
'rotten'+'row'='ROTTEN ROW'
Can you help me to learn more?
(Other definitions for rotten row that I've seen before include "Capital place for riding" , "comes to an end at Hyde Park Corner" , "Once fashionable avenue for riders in London" , "Part of London park" , "Where to see riders" .)