Could you say flying is frightening? (5)
I believe the answer is:
awing
I believe this clue is a double definition.
'could you say flying' is the first definition.
(awing or a-wing can mean flying)
'frightening?' is the second definition.
(awing is a kind of frightening)
'is' is the link.
(Other definitions for awing that I've seen before include "inspiring great admiration" , "inspiring with dread" , "having dreadful effect" , "Causing dread" , "Causing reverent fear" .)