Child runs with unknown soldiers (8)
I believe the answer is:
infantry
'soldiers' is the definition.
(I've seen this before)
'child runs with unknown' is the wordplay.
'child' becomes 'infant' (infant is a kind of child).
'runs' becomes 'r' (cricket abbreviation).
'with' means one lot of letters go next to another.
'unknown' becomes 'y' (algebra).
'infant'+'r'+'y'='INFANTRY'
(Other definitions for infantry that I've seen before include "Men fighting" , "Army unit of foot soldiers" , "Troops" , "Grunts" , "Unmounted fighters" .)