Sinister English upset Ireland (5)
I believe the answer is:
eerie
'sinister' is the definition.
(I've seen this before)
'english upset ireland' is the wordplay.
'english' becomes 'E' (abbreviation).
'upset' says the letters should be written in reverse.
'ireland' becomes 'eire' (poetic name for Ireland).
'eire' written backwards gives 'erie'.
'e'+'erie'='EERIE'
(Other definitions for eerie that I've seen before include "Ghostly" , "Uncannily strange" , "Strangely frightening" , "Weird and disturbing" , "Bloodcurdling" .)