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' shows that the letters should be reversed in order.
'ireland' becomes 'eire' (poetic name for Ireland).
'eire' reversed gives 'erie'.
'e'+'erie'='EERIE'
(Other definitions for eerie that I've seen before include "Ghostly" , "Uncannily strange" , "Strangely frightening" , "Weird and disturbing" , "Bloodcurdling" .)