Think up (6)
I believe the answer is:
invent
'think up' is the definition.
(I know that invent is a kind of the think up)
This is all the clue.
(Other definitions for invent that I've seen before include "Design for the first time" , "Be the first person to make something" , "Create like Edison" , "Originate" , "Make up (something false)" .)