No good chasing Henrietta — terribly scary (11)
I believe the answer is:
threatening
'scary' is the definition.
(I've seen this before)
'no good chasing henrietta terribly' is the wordplay.
'no good' becomes 'ng' (abbreviation).
'chasing' is a charade indicator (letters next to each other) (some letters chase after others).
'terribly' is an anagram indicator.
'henrietta' with letters rearranged gives 'threateni'.
'ng' put after 'threateni' is 'THREATENING'.
(Other definitions for threatening that I've seen before include "Scary" , "Putting at risk" , "Dangerous" , "giving a warning for the future" , "Minatory" .)