Regret seizing film in train (7)
I believe the answer is:
retinue
'train' is the definition.
(people following someone in a procession)
'regret seizing film in' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'seizing' means one lot of letters goes inside another (inserted letters are seized).
'film' becomes 'ET' (E.T. the Extra-Terrestrial).
'et'+'in'='etin'
'rue' going around 'etin' is 'RETINUE'.
(Other definitions for retinue that I've seen before include "Reunite (anag) - group of servants" , "Suite - train" , "Train, escort" , "Body of aides" , "Cortege" .)