Grunts of young one on the line (8)
I believe the answer is:
infantry
'grunts' is the definition.
(I've seen this before)
'young one on the line' is the wordplay.
'young' becomes 'infant' (similar in meaning).
'one on' says to put letters next to each other.
'the line' becomes 'ry' (abbreviation for railway).
'infant'+'ry'='INFANTRY'
'of' is the link.
(Other definitions for infantry that I've seen before include "Army unit of foot soldiers" , "part of the army" , "Unmounted fighters" , "Grunts" , "Men fighting" .)