Regret importing European metal train (7)
I believe the answer is:
retinue
'train' is the definition.
(thesaurus)
'regret importing european metal' is the wordplay.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'importing' means one lot of letters goes inside another.
'european' becomes 'e' (abbreviation e.g. EU).
'metal' becomes 'tin' (tin is a kind of metal**).
'e'+'tin'='etin'
'rue' placed around 'etin' is 'RETINUE'.
(Other definitions for retinue that I've seen before include "Body of followers" , "Group of followers" , "Train; attendants" , "Body of attendants" , "Figure out" .)