Train to put into words (7)
I believe the answer is:
express
This clue is a double definition.
'train' is the first definition.
(express train is a kind of train)
'put into words' is the second definition.
(I've seen this before)
'to' is the link.
(Other definitions for express that I've seen before include "Intimate - squeeze out" , "High-speed; specific" , "air" , "Fleet" , "Transport" .)