Conspiring old American sounds a warning (2,7)
I believe the answer is:
in cahoots
'conspiring' is the definition.
(to be in cahoots with someone is to plot with them)
'old american sounds a warning' is the wordplay.
'old american' becomes 'Inca' (the Inca were a South America people).
'sounds a warning' becomes 'hoots' (a hoot might be used as a warning).
'inca'+'hoots'='IN CAHOOTS'
(Other definitions for in cahoots that I've seen before include "what conspirators are" , "Thick" , "Associated with" , "Working with" , "Colluding" .)