Youngster with extremely rowdy soldiers (8)
I believe the answer is:
infantry
'soldiers' is the definition.
(I've seen this before)
'youngster with extremely rowdy' is the wordplay.
'youngster' becomes 'infant' (I've seen this before**).
'with' says to put letters next to each other.
'extremely' says to hollow out the word (remove centre letters).
'rowdy' with its middle removed is 'ry'.
'infant'+'ry'='INFANTRY'
(Other definitions for infantry that I've seen before include "Foot-soldiers" , "Unmounted fighters" , "Army unit of foot soldiers" , "Troops" , "part of the army" .)