One ruining device that lowers lift (7)
I believe the answer is:
spoiler
I believe this is a double definition.
'one ruining' is the first definition.
'device that lowers lift' is the second definition.
(Other definitions for spoiler that I've seen before include "Device on plane or vehicle to slow it down (7)" , "Destructive person" , "Aerodynamic surface on car" , "attachment to racing car" , "News story published to ruin another" .)