Strangely frightening -- topless noble, that is (5)
I believe the answer is:
eerie
'strangely frightening' is the definition.
(I've seen this before)
'topless noble that is' is the wordplay.
'topless' means to remove the first letter.
'noble' becomes 'peer' (peer is a kind of noble).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'peer' with its first letter taken off is 'eer'.
'eer'+'ie'='EERIE'
(Other definitions for eerie that I've seen before include "Awesome" , "Spooky, supernatural" , "Unnatural" , "Sinister" , "Scary, weird" .)