Youth needs one place in train (9)
I believe the answer is:
stripling
'youth' is the definition.
(I've seen this before)
'needs one place in train' is the wordplay.
'needs one place' becomes 'pli' (I can't explain this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'train' becomes 'string' ('string' can be a synonym of 'train').
'pli' put into 'string' is 'STRIPLING'.
Can you help me to learn more?
(Other definitions for stripling that I've seen before include "Young man" , "Young boy" , "Youth" , "youngster" , "Young 'un" .)