River, south-eastern? Wrong (5)
I believe the answer is:
false
'wrong' is the definition.
('false' can be similar in meaning to 'wrong')
'river south-eastern?' is the wordplay.
'river' becomes 'fal' (I've seen this before**).
'south eastern?' becomes 'se'.
'fal'+'se'='FALSE'
(Other definitions for false that I've seen before include "Not true" , "Erroneous" , "Not in accordance with fact" , "Forged" , "Not genuine or real" .)