Pop back with train set (7)
I believe the answer is:
appoint
'set' is the definition.
(thesaurus)
'pop back with train' is the wordplay.
'pop' becomes 'pa' (both can mean father).
'back' shows that the letters should be reversed in order.
'with' means one lot of letters go next to another.
'train' becomes 'point' (pointing is a kind of training**).
'pa' written backwards gives 'ap'.
'ap'+'point'='APPOINT'
(Other definitions for appoint that I've seen before include "Put into a post" , "Furnish" , "Assign to a duty" , "Pick" , "Fix" .)