Unpleasant argument in Hyde Park (6,3)
I believe the answer is:
rotten row
'in hyde park' is the definition.
(I know that Hyde Park has part or parts Rotten Row)
'unpleasant argument' is the wordplay.
'unpleasant' becomes 'rotten' (I've seen this before).
'argument' becomes 'row' (to row is to argue).
'rotten'+'row'='ROTTEN ROW'
(Other definitions for rotten row that I've seen before include "Once fashionable avenue for riders in London" , "Horses here" , "Where to see riders" , "Track through Hyde Park" , "Part of London park" .)