Ireland rising under English is strangely frightening (5)
I believe the answer is:
eerie
'strangely frightening' is the definition.
(I've seen this before)
'ireland rising under english' is the wordplay.
'ireland' becomes 'eire' (poetic name for Ireland).
'rising' is a reversal indicator (going up in a down clue).
'under' says to put letters next to each other (in a down clue, some letters go below others).
'english' becomes 'E' (abbreviation).
'eire' written backwards gives 'erie'.
'erie' put after 'e' is 'EERIE'.
'is' acts as a link.
(Other definitions for eerie that I've seen before include "Weird, uncanny" , "Spooky, supernatural" , "Unnerving" , "Mysterious" , "Scary, weird" .)