Turn upside down yet remain vertical essentially (6)
I believe the answer is:
invert
'turn upside down' is the definition.
('invert' can be a synonym of 'turn upside down')
'yet remain vertical essentially' is the wordplay.
'essentially' says the answer is hidden in the clue (essence can mean the inside of something).
'INVERT' can be found hidden inside 'yet remain vertical'.
(Other definitions for invert that I've seen before include "Turn inside out, reverse position" , "Upset" , "Turn upside down, reverse order" , "Turn inside out or upside down" , "Upend" .)