Scary nun going round Northern Ireland (8)
I believe the answer is:
sinister
'scary' is the definition.
(I know this)
'nun going round northern ireland' is the wordplay.
'nun' becomes 'sister' (a nun might be called sister).
'going round' is an insertion indicator (some letters go around others).
'northern ireland' becomes 'NI' (abbreviation).
'sister' going around 'ni' is 'SINISTER'.
(Other definitions for sinister that I've seen before include "Disturbing" , ", nun going round home" , "Eerie, alarming" , "Threatening evil on the heraldic left" , "not right!" .)