Regret accepting European money for train (7)
I believe the answer is:
retinue
'train' is the definition.
(thesaurus)
'regret accepting european money' is the wordplay.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'accepting' indicates putting letters inside.
'european' becomes 'e' (abbreviation e.g. EU).
'money' becomes 'tin' (I've seen this before**).
'e'+'tin'='etin'
'rue' going around 'etin' is 'RETINUE'.
'for' acts as a link.
(Other definitions for retinue that I've seen before include "Reunite (anag) - group of servants" , "Body of retainers" , "Attendants, followers" , "Body of followers" , "Figure out" .)