Sioux pick up good inside information (6)
I believe the answer is:
'sioux' is the definition.
'pick up good inside information' is the wordplay.
'pick up' says the letters should be written in reverse.
'good' becomes 'ok'.
'inside' means one lot of letters goes inside another.
'information' becomes 'data' (I've seen this before).
'ok' reversed gives 'ko'.
'ko' inserted into 'data' is 'DAKOTA'.
(Other definitions for dakota that I've seen before include "Douglas DC 3 name; N. American native", "US territory split North and South", "Mass-produced transport plane, WWII workhorse, the DC-3", "North American Indian; Douglas DC-3", "Former US territory, now divided into two states".)