Regulars in army supporting child soldiers (8)
I believe the answer is:
infantry
'soldiers' is the definition.
(I've seen this before)
'regulars in army supporting child' is the wordplay.
'regulars in' indicates alternate letters.
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'child' becomes 'infant' (infant is a kind of child**).
The alternating letters of 'army' are 'ry'.
'ry' after 'infant' is 'INFANTRY'.
(Other definitions for infantry that I've seen before include "Troops" , "Men fighting" , "Unmounted fighters" , "Army unit of foot soldiers" , "part of the army" .)