Took train north for first time (5)
I believe the answer is:
debut
'first time' is the definition.
(eg a debut performance)
'took train north' is the wordplay.
'took' becomes 'd' (I can't explain this - if you can you should give a lot more credence to this answer).
'train' becomes 'tube' (tube is a type of subway train).
'north' shows that the letters should be reversed in order (in a down clue, letters go up or 'northward').
'tube' in reverse letter order is 'ebut'.
'd'+'ebut'='DEBUT'
'for' is the link.
Can you help me to learn more?
(Other definitions for debut that I've seen before include "Appear in public for the first time" , "First appearance in society" , "opening" , "First public appearance of performer" , "The coming out of a society girl" .)