River mouth reconstructed in South America after death (10)
I believe the answer is:
posthumous
'after death' is the definition.
(I've seen this before)
'river mouth reconstructed in south america' is the wordplay.
'river' becomes 'Po' (Italian river).
'reconstructed' indicates anagramming the letters.
'in' indicates putting letters inside.
'south' becomes 's' (abbreviation).
'america' becomes 'us' (United States).
'mouth' with letters rearranged gives 'thumo'.
's'+'us'='sus'
'thumo' going into 'sus' is 'sthumous'.
'po'+'sthumous'='POSTHUMOUS'
(Another definition for posthumous that I've seen is " Occurring after death".)