Regret having base metal in train (7)
I believe the answer is:
retinue
'train' is the definition.
(I've seen this before)
'regret having base metal' is the wordplay.
'regret' becomes 'rue' (synonyms).
'having' indicates putting letters inside.
'base' becomes 'e' (Euler's number - base of natural logarithms).
'metal' becomes 'tin' (tin is a kind of metal).
'e'+'tin'='etin'
'rue' placed around 'etin' is 'RETINUE'.
'in' is the link.
(Other definitions for retinue that I've seen before include "Group following an important person" , "Suite of followers" , "Body of followers" , "Group of followers" , "Train, following" .)