So the soldiers must go over front (9)
I believe the answer is:
therefore
'so' is the definition.
(I've seen this before)
'the soldiers must go over front' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'must go over' means one lot of letters go next to another (in a down clue, some letters go above others).
'front' becomes 'fore' (fore is a kind of front).
'the'+'re'+'fore'='THEREFORE'
(Other definitions for therefore that I've seen before include "Ergo" , "Thus" , "Consequently - for that reason" , "In consequence" , "It follows" .)