Bitterly regret carrying money on European train (7)
I believe the answer is:
retinue
'train' is the definition.
(thesaurus)
'bitterly regret carrying money on european' is the wordplay.
'bitterly regret' becomes 'rue' (I've seen this before).
'carrying' is an insertion indicator.
'money' becomes 'tin' (I've seen this before**).
'on' is a charade indicator (letters next to each other).
'european' becomes 'e' (abbreviation e.g. EU).
'tin' after 'e' is 'etin'.
'rue' going around 'etin' is 'RETINUE'.
(Other definitions for retinue that I've seen before include "Body of attendants" , "Group attending a person" , "Group following an important person" , "Train, following" , "Suite of followers" .)