Endanger that erne fluttering (8)
I believe the answer is:
threaten
'endanger' is the definition.
(I know that endanger can be written as threaten)
'that erne fluttering' is the wordplay.
'fluttering' indicates anagramming the letters.
'that'+'erne'='thaterne'
'thaterne' is an anagram of 'THREATEN'.
(Other definitions for threaten that I've seen before include "Look likely to" , "indicate danger" , "Give warning of something unpleasant" , "State an intention to harm" , "Utter intentions of injury or punishment" .)