Frightening a fly (5)
I believe the answer is:
awing
'frightening' is the definition.
(awing is a kind of frightening)
'a fly' is the wordplay.
'fly' becomes 'wing' ('wing' can be a synonym of 'fly').
'a'+'wing'='AWING'
(Other definitions for awing that I've seen before include "having dreadful effect" , "making an impression" , "inspiring great admiration" , "Inspiring reverence" , "producing reverence" .)