Invent lies I've decided to keep up (6)
I believe the answer is:
devise
'invent' is the definition.
(I know that invent can be written as devise)
'lies i've decided to keep up' is the wordplay.
'to keep' says the answer is hidden in the clue.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'esived' can be found hidden inside 'lies ive decided'.
'esived' in reverse letter order is 'DEVISE'.
(Other definitions for devise that I've seen before include "Create, work out" , "Contrive" , "Plan invent" , "Come up with, a plan perhaps" , "Formulate or plan" .)