Early South American period, included a large number in private (2,6)
I believe the answer is:
in camera
'in private' is the definition.
(I've seen this before)
'early south american period included a large number' is the wordplay.
'early south american' becomes 'inca' (I've seen this in another clue).
'period' becomes 'era' (era is a kind of period).
'included' is an insertion indicator.
'a large number' becomes 'm'.
'inca'+'era'='incaera'
'incaera' enclosing 'm' is 'IN CAMERA'.
(Other definitions for in camera that I've seen before include "not for all to see" , "not shown to the public?" , "Where you find digital pictures" , "secret" , "Conducted in private" .)