Train seat he occupies someone vacated (8)
I believe the answer is:
rehearse
'train' is the definition.
(eg actors rehearse to train for a show)
'seat he occupies someone vacated' is the wordplay.
'seat' becomes 'rear' (synonyms).
'occupies' means one lot of letters goes inside another.
'vacated' means to remove the middle letters.
'someone' with its middle taken out is 'se'.
'rear' enclosing 'he' is 'rehear'.
'rehear'+'se'='REHEARSE'
(Other definitions for rehearse that I've seen before include "Practise before performing in public" , "Practise before a public performance" , "Go over" , "What actor does to prepare for play" , "Recite - try out" .)