Small child taking railway from foot-soldiers (6)
I believe the answer is:
infant
'small child' is the definition.
(I've seen this before)
'railway from foot-soldiers' is the wordplay.
'railway' becomes 'ry' (abbreviation for railway).
'from' is a deletion indicator.
'foot-soldiers' becomes 'infantry' (I've seen this before).
'infantry' with 'ry' removed is 'INFANT'.
'taking' is the link.
(Other definitions for infant that I've seen before include "Tiny baby" , "Thing in early stage; minor (legal)" , "Wee one" , "Juvenile" , "Very young child or baby" .)