Documentaries about losing minors' letters in train (7)
I believe the answer is:
educate
'train' is the definition.
('educate' can be a synonym of 'train')
'documentaries about losing minors' letters' is the wordplay.
I cannot quite see how this works, but
'about' could be 'ca' (abbreviation for 'circa') and 'ca' is found within the answer.
This may be the basis of clue (or it may be nonsense).
'in' acts as a link.
Can you help me to learn more?
(Other definitions for educate that I've seen before include "Teach - instruct" , "Enlighten" , "what teacher should do, to be effective" , "Impart knowledge" , "Impart learning" .)