Horrify a European on the way back (5)
I believe the answer is:
appal
'horrify' is the definition.
('appal' can be a synonym of 'horrify')
'a european on the way back' is the wordplay.
'european' becomes 'lapp' (Lapp is an example**).
'on the way back' is a reversal indicator.
'lapp' back-to-front is 'ppal'.
'a'+'ppal'='APPAL'
(Other definitions for appal that I've seen before include "Disgust" , "Cause to be unpleasantly surprised" , "Dismay, horrify" , "To shock, dismay" , "Shock, horrify" .)