Diaphanous or very steep (5)
I believe the answer is:
sheer
This clue is a double definition.
'diaphanous' is the first definition.
(I know that diaphanous can be written as sheer)
'very steep' is the second definition.
(I've seen this in another clue)
(Other definitions for sheer that I've seen before include "Very steep - may apply to 4" , "Absolutely - fine - straight up" , "Very fine like silk stockings" , "Unmitigated; perpendicular" , "Absolutely - diaphanous" .)