Great Britain's flipping harsh (5)
I believe the answer is:
acerb
'harsh' is the definition.
(thesaurus)
'great britain's flipping' is the wordplay.
'great' becomes 'ace' (I've seen this before).
'britain' becomes 'br' (abbreviation).
'flipping' shows that the letters should be reversed in order.
'br' reversed gives 'rb'.
'ace'+'rb'='ACERB'
(Other definitions for acerb that I've seen before include "sharp" , "tart" , "Sour, bitter" , "harsh and bitter" .)