Colluding, old S American riots (2,7)

I believe the answer is:

in cahoots

'colluding' is the definition.
(I've seen this before)

'old s american riots' is the wordplay.
'old s american' becomes 'inca' (I've seen this before).
'riots' becomes 'hoots'.
'inca'+'hoots'='IN CAHOOTS'

(Other definitions for in cahoots that I've seen before include "Associated with" , "Conniving" , "Working with" , "what conspirators are" , "Colluding" .)

I've seen this clue in The Independent.

Want a hint initially instead of a full solution? Install my app