Lines on baby's foot (8)
I believe the answer is:
infantry
'foot' is the definition.
(I know that foot can be written as infantry)
'lines on baby's' is the wordplay.
'lines' becomes 'ry' (abbreviation for railway).
'on' says to put letters next to each other.
'baby' becomes 'infant' (synonyms).
'ry' after 'infant' is 'INFANTRY'.
(Other definitions for infantry that I've seen before include "Unmounted fighters" , "Army unit of foot soldiers" , "Foot-soldiers" , "Grunts" , "fighting group" .)