Lack of sense sets back European political group (12)
I believe the answer is:
stupefaction
'lack of sense' is the definition.
'sets back european political group' is the wordplay.
'sets' becomes 'puts' ('put' can be a synonym of 'set').
'back' shows that the letters should be reversed in order.
'european' becomes 'e' (abbreviation e.g. EU).
'political group' becomes 'faction'.
'puts' backwards is 'stup'.
'stup'+'e'+'faction'='STUPEFACTION'
(Other definitions for stupefaction that I've seen before include "Extreme astonishment" , "great astonishment" , "Total astonishment and shock" , "Shocked astonishment" .)