Difficult area within South American river's mouth part (4,6)

'mouth part' is the definition.
('hard palate' is part of 'mouth')

'difficult area within south american river's' is the wordplay.
'difficult' becomes 'hard' ('hard' can be a synonym of 'difficult').
'area' becomes 'a' (maths abbreviation).
'within' indicates putting letters inside.
'south american river' becomes 'plate' (I've seen this before).
'a' inserted within 'plate' is 'palate'.
'hard'+'palate'='HARD PALATE'