Flying is stunning (5)
I believe the answer is:
awing
I believe this is a double definition.
'flying' is the first definition.
(rare adverb, sometimes spelled a-wing)
'is stunning' is the second definition.
(to awe is to stun or astonish)
(Other definitions for awing that I've seen before include "Causing reverent fear" , "inspiring with dread" , "Creating reverence" , "Causing dread" , "producing reverence" .)