Yonder, the Royal Engineers (5)
I believe the answer is:
there
'yonder' is the definition.
(I've seen this before)
'the royal engineers' is the wordplay.
'royal engineers' becomes 're'.
'the'+'re'='THERE'
(Other definitions for there that I've seen before include "In that respect" , "Triumphant exclamation" , "Yonder" , "Place: as indicated" , "In that location" .)