Soldiers with major are outstanding (6)
I believe the answer is:
remain
'are outstanding' is the definition.
(eg outstanding bills are bills which remain)
'soldiers with major' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'with' is a charade indicator (letters next to each other).
'major' becomes 'main' (both can mean important).
're'+'main'='REMAIN'
(Other definitions for remain that I've seen before include "Stay (in position)" , "Last" , "stop behind?" , "Don't leave (the EU?)" , "Stay or continue to be" .)