Child runs unknown part of army (8)
I believe the answer is:
infantry
'army' is the definition.
The answer and definition can be both groups as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'child runs unknown' is the wordplay.
'child' becomes 'infant' (infant is a kind of child).
'runs' becomes 'r' (cricket abbreviation).
'unknown' becomes 'y' (algebra).
'infant'+'r'+'y'='INFANTRY'
'part of' acts as a link.
Can you help me to learn more?
(Other definitions for infantry that I've seen before include "fighting group" , "Men fighting" , "Foot-soldiers" , "part of the army" , "Troops" .)