Dark American following number on Spanish river (9)

'dark' is the definition.
('tenebrous' can be similar in meaning to 'dark')

'american following number on spanish river' is the wordplay.
'american' becomes 'us'.
'following' is a charade indicator (letters next to each other).
'number' becomes 'ten' (ten is a kind of number**).
'on' means one lot of letters go next to another.
'spanish river' becomes 'ebro' (I've seen this before).
'ten'+'ebro'='tenebro'
'us' put after 'tenebro' is 'TENEBROUS'.