Plotting together? Old American laughs (2,7)
I believe the answer is:
in cahoots
'plotting together?' is the definition.
(people secretly plotting are in cahoots)
'old american laughs' is the wordplay.
'old american' becomes 'Inca' (the Inca were a South American people).
'laughs' becomes 'hoots'.
'inca'+'hoots'='IN CAHOOTS'
(Other definitions for in cahoots that I've seen before include "what conspirators are" , "Thick" , "Conniving" , "Conspiring together" , "Colluding" .)