Soldier waits on reinforcements (8)
I believe the answer is:
'reinforcements' is the definition.
'soldier waits on' is the wordplay.
'soldier' becomes 're' (Royal Engineer).
'waits on' becomes 'serves' (to wait on someone is to serve them).
(Other definitions for reserves that I've seen before include "Stockpiles" , "group of people for backup?" , "Books" .)