Mishap ultimately stops new girl with Angus making European festival (9,5)
I believe the answer is:
walpurgis night
'european festival' is the definition.
(festival celebrated in many European countries)
'mishap ultimately stops new girl with angus' is the wordplay.
'ultimately' says to take the final letters.
'stops' is an insertion indicator.
'new' is an anagram indicator.
The last letter of 'mishap' is 'p'.
'girl'+'with'+'angus'='girlwithangus'
'girlwithangus' with letters rearranged gives 'walurgisnight'.
'p' going into 'walurgisnight' is 'WALPURGIS NIGHT'.
'making' is the link.
(Other definitions for walpurgis night that I've seen before include "30th April?" , "Witches' celebration" , "enchanting event?" , "May Day's eve (from German folklore)" , "One evening" .)