Billion invested in basic automatic device (5)
I believe the answer is:
robot
'automatic device' is the definition.
'billion invested in basic' is the wordplay.
'billion' becomes 'b'.
'invested in' means one lot of letters goes inside another.
'basic' becomes 'root' (similar in meaning).
'b' placed into 'root' is 'ROBOT'.
(Other definitions for robot that I've seen before include "Man-made man - man-made machine" , "Lifelike machine" , "performer lacking emotion" , "Mechanism that can move like a human" , "Mechanical worker" .)