Clash with frightful European clutching flag (8)
I believe the answer is:
disagree
'clash with' is the definition.
(thesaurus)
'frightful european clutching flag' is the wordplay.
'frightful' becomes 'dire' (I've seen this before).
'european' becomes 'e' (abbreviation e.g. EU).
'clutching' means one lot of letters goes inside another.
'flag' becomes 'sag' (synonyms).
'dire'+'e'='diree'
'diree' going around 'sag' is 'DISAGREE'.
(Other definitions for disagree that I've seen before include "Be at variance" , "Differ, take issue" , "Express a contrary view" , "Dire ages (anag.)" , "Be at odds" .)