Nobleman blowing top -- that is strangely frightening (5)
I believe the answer is:
eerie
'strangely frightening' is the definition.
(I've seen this before)
'nobleman blowing top that is' is the wordplay.
'nobleman' becomes 'peer' (peer is a kind of nobleman).
'blowing top' means to remove the first letter.
'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 "Strange, unusual" , "Bloodcurdling" , "Spine-tingling" , "Spooky, creepy" , "Strangely frightening" .)