Vertical deviation (5)
I believe the answer is:
sheer
This clue is a double definition.
'vertical' is the first definition.
(I've seen this before)
'deviation' is the second definition.
(I've seen this before)
(Other definitions for sheer that I've seen before include "Absolutely - fine - straight up" , "Plumb" , "Deviate (from) - vertical" , "Downright - deviation" , "transparent fabric" .)