Decline getting into calamitous European fight (8)
I believe the answer is:
disagree
'fight' is the definition.
(thesaurus)
'decline getting into calamitous european' is the wordplay.
'decline' becomes 'sag' (sag can mean to decline or droop).
'getting into' means one lot of letters goes inside another.
'calamitous' becomes 'dire' (I've seen this in another clue).
'european' becomes 'E' (abbreviation e.g. EU).
'sag' put into 'dire' is 'disagre'.
'disagre'+'e'='DISAGREE'
(Other definitions for disagree that I've seen before include "Be at odds" , "Have words" , "Quarrel" , "Differ, argue" , "Hold different opinions" .)