Demote soldiers in front of emissary (8)
I believe the answer is:
relegate
'demote' is the definition.
('relegate' can be a synonym of 'demote')
'soldiers in front of emissary' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'in front of' means one lot of letters go next to another.
'emissary' becomes 'legate' (legate is a kind of emissary).
're'+'legate'='RELEGATE'
(Other definitions for relegate that I've seen before include "Put in a lower division" , "Move to a lower division" , "Send down" , "Transfer to a lower division" , "Move to a lower position" .)