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