American had one month on the river (5)

'american' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'one month on the river' is the wordplay.
'one month' becomes 'Sep' (abbreviation for September).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'the river' becomes 'Po' (Italian river).
'sep'+'po'='SEPPO'

'had' acts as a link.

Can you help me to learn more?